If you're considering buying a new camera though, consider taking a look at the rest of your equipment set up first. This isn't going to be one of those patronising posts where I make assumptions about your skill level and tell you that you should learn photography properly and forget about buying a new camera (seriously, why do so many photography bloggers insult their readers by assuming everyone is just a gear head with no skills?). By all means get a new camera if you feel you're ready but most modern cameras, even entry level ones are very capable and a well rounded set of lenses and accessories can be a better investment, especially if you're missing some of the basics.
So what then do I consider is essential for your bag? Lets look at the basics first and then look at some additional things that you might want to consider.
A Good Tripod
Shot on a Tripod
I've put this first, because a surprising number of photographers don't have a decent tripod. I know that there are quite a few people who don't like shooting on a tripod or feel it interferes with their art, but you really should have a good tripod. If you want to take images that are tack sharp, or if you want to blur car lights for example, or shoot at night properly you need a tripod. Even if you think you don't, or that you can just raise the ISO and shoot hand held, once you've seen the possibilities of shooting with a tripod, and see the advantages, you'll understand why it's so important.
A good tripod can be an investment, but the difference between a good one and a mediocre one is huge. If nothing else get a descent ball head with standard legs. A ball head is one of those things you'll wonder how you did without once you get one. It makes a huge difference and makes shooting with a tripod much more pleasurable. I have a manfrotto hydrostatic ball head and it's pretty good.
In terms of legs, you need something sturdy as a start, and optionally something light. The other thing to look for is the locking mechanism. My first set of tripod legs, which was a cheap pair of manfrotto aluminium legs, had a thumbscrew type locking mechanism, and now they're very difficult to lock properly because the mechanism has worn. It's not that old either. You have to twist the screw really hard now to get it to lock, and it's a real pain. The more expensive tripods have either twist and lock mechanisms or a lever type lock. Either works pretty well. Spending a bit of money now on a good set of tripod legs and a good tripod head will be a worth while investment because a good one should last for years.
Here are a few lenses for your kit that you should consider if you don't already have them.
The first is a good telephoto. If you can afford a pro level one then by all means, but a lot of the semi-pro telephoto lenses are pretty good. On the Nikon side the 70-300mm is apparently a really good lens (I have a Sigma 70-200 2.8 which you can see my review of here) and on the canon side the 70-200 f4 is relatively inexpensive, but very good.
Pro Level Standard Zoom
The second thing you should consider is getting a pro level standard zoom. Why? Well, every now and then you will come across a situation where you need to take a photo for someone that absolutely has to be the best quality possible. If you have at least one pro level lens you'll always have the capability at least. Yo don't need the highest end lens either, but something in the high end will serve you incredibly well. And if you have just one pro level lens, having it be a standard zoom will probably be the most versatile. You can expand out your range in the future but this is something I would consider essential if you want to expand your photography capabilities. They can be expensive, but what you get from a good pro level lens will be worth more in the long run than spending the same amount on a camera body.
For full frame: On Canon, either the 24-105 or the 24-70. On Nikon, either the 24-120 F4 (not the previous generation) or the 24-70 if you can afford it. If you are on a cropped sensor camera consider either manufacturer's 17-55 f2.8.
Again, you might think this is excessive if you are beginning photography but any of these lenses will allow you to take images with the technical quality to satisfy the most demanding client (provided your technique is up to scratch of course!)
Shot with a 10-22mm Lens
This is another one of those lenses that people don't think they need until they get one and then realise what they've been missing out on. If you want to take landscapes or cityscapes, having an ultra wide will make a huge difference. Luckily there are a few good ones to choose from in various price points. For full frame, Canon has the excellent 17-40F4L and 16-35L (which is quite expensive). Nikon has the expensive but (apparently) amazing 14-24 f/2.8 and the very versatile and less expensive 16-35f4 VR. Again, both companies have cropped sensor versions.
Shot on a Nikon D700 with 50mm 1.8
You should get a 50mm if you don't already have one (or a 35mm on a cropped sensor) The reason is two fold. These standard lenses are pretty inexpensive and the quality to cost ratio is enormous. The reason to have a standard lens (as they're called) is because it's the focal length that most closely matches what the human eye sees. Shooting with a 50mm prime is quite liberating, and it really teaches you to think when composing shots. Some people shoot with nothing but a 50, (which I don't agree with), but for a lot of subjects, particularly street photography it's a prefect lens, so you really should have one in your bag. A 50mm 1.8 will set you back around €200 so it's a pretty cheap investment relatively speaking for the payoff.
Shot using an off camera Canon flash
I hear the phrase "I don't like the look of flash, I prefer to shoot in natural light" a lot from photographers of various skill levels. I don't mean to sound rude or patronising, but if you believe that, it's because you don't know how to shoot properly with flash. Why? The thing with flash is if you use it right it won't look like flash. It will look like natural light. The advantage is it gives you complete control over that light. Once you do it will change your photography forever. Here's the thing, if you are in doubt, do me a favour. Go look at the work of Joe McNally. Seriously, do it now, I'll wait. Ok, you're back! See what I mean?
Anyway, you should have the capability to use flash. And to use it properly you should be able to use your flash off your camera. This is a huge subject, and I won't go into it in detail here but I would recommend the mid range or top end flash from both Canon and Nikon although I'd steer clear of the basic models as they don't have all the features you need to shoot off camera. If you want to learn more check out Joe McNally's classes over at Kelby Training (seriously, you won't regret it) or check out Strobist.com.
Here are a few things that I wouldn't consider essential, but are pretty handy to have
I'll call this a travel lens, but what I really ran is a "super zoom" to use the popular term. These are really handy to have because while they might not be perfect in terms of optical quality, the advantage of not having to change lens can be really useful in certain circumstances. It also makes it really handy if you need to travel light.
I have the excellent Nikon 28-300 on my D700. It's a great lens. It's pretty sharp all round, and while it's not the fastest glass, the flexibility is worth it. I always use this when travelling, and I've gotten some of my best images with it.
Before I had the 28-300, I had it's cropped sensor little brother the 18-200, which has now been superseded by the 18-300. this was also a great lens, although in my opinion, not as optically good as the 28-300.
I'm not as familiar with the Canon equivalent of the 28-300 but it's much more expensive than the Nikon version. It's €3000 here in Ireland compared to under €1000 for the nikon version. It's also a pretty big lens.
The other lens to consider is getting a good macro. I have a macro lens on both my Canon and Nikon systems (I have both systems - long story) and I love shooting macro. My Nikon lens is an old one that I got second hand, and it's before the VR version came out. It's a great lens but it's long gone from production, so there's not much point in me recommending that!
My Canon one is the previous generation 105mm Macro and it's a superb lens. It's probably the sharpest lens I have on any body.
So that's about it. That's my list of what I think are essentials for any camera bag. I'm sorry for only focussing on Nikon and Canon, but they're the brands I'm familiar with. If you have you're own suggestions please leave them in the comments below, but please remember this isn't about the philosophy of gear vs art so please keep it relevant.