The combination I recommend for amateur usage:
- you need good legs (stable, high enough, solid, weather resistant) - my favorite at a reasonable price: Manfrotto 055PROB (in America the same tripod is sold under another reference: 3021 if I am not wrong). Manfrotto also has a high end carbon version with a quick-release mechanism. This is what everybody wants, but I found it a bit expensive if you don't shoot systematically on a tripod.
- you need a good ball-head with a quick-release mechanism - I recommend the grip ballhead of Manfrotto, very easy to manipulate in all directions. Supports up to 5-6 kg (a pro-body and a serious tele-lens). I have modified the grip to be able to use it with my left hand, while reaching the buttons on the camera with my right hand (takes 10 minutes with the tools provided).
- the grip ball-head I am mentioning has a disadvantage: it cannot be used for panning. To do panning (panorama - stitching pictures together for instance), you need to be able to lock two axis so that you can move the camera in a single plane. You'll need another type of head.
The picture hereunder gives a view of the total panorama combo: the grip ballhead mounted on a 300N Pano head (allowing to take successive picture at a constant angle (selectable)). It is of course necessary to lock the ballhead in a horizontal position using the integrated bubble level.
- for other occasions and when traveling light, I am also using a monopod (typically for sports shooting), a table top mini tripod, the Manfrotto 345, a macro bracket allowing to install two flashes on both sides of the camera, also coming from Manfrotto (3278) and a small Manfrotto ball-head, very light and easier to transport but with a maximal charge of 2.5kg (i.e. D200+grip+ short lens)
More information to be found on http://www.manfrotto.com
There are other excellent manufacturers making tripods: one of the popular ones is Gitzo, there is also Velbon (a bit cheaper and lower end but good enough for many amateurs)