Tripods
If you want to immediately improve your photography, the next purchase you should make after buying the camera is to purchase a tripod. I've used both Bogen and Gitzo models. For pure speed, the Bogen leg clamps are much faster than the Gitzo twist-locks but I've never had an issue with either.
I would recommend a model that does not have a center post for two reasons. First, a center post prevents you from placing the tripod close to the ground (for those macro shots of mushrooms or poppy's). Second, I often see it used to increase the vertical height of the camera and your just asking for vibration problems when doing this - raise the entire height of the tripod instead. (Yes, I know using it is better than not getting the shot at all.)
For metal tripods, go to Home Depot and buy a section of closed-cell foam pipe insulation and a roll of duct tape. Cut the foam tubing to fit the top leg section and secure it with the duct tape. It keeps your hands from having to touch the freezing metal on those cold winter mornings.
Bags
Bags seem to be every photographer's weakness. Like the search for the best light, there is a neverending quest for the perfect bag (which I've come to believe does not exist.)
I like outdoor photgraphy and for me, backpacks are a good fit. If you find something that works for you - use it! Like any other piece of gear you need to know your bag. When the light is fading fast, or you need to change lenses in a hurry, you don't want to be fumbling with your bag. Have a system where you keep things.
The most important piece of advice I can pass along is to always zip or close your bag. I nearly dropped all my gear due to leaving the cover unzipped and while in a hurry, grabbing the pack out of the back of my car. A frantic grab for the cover saved my precious few lenses and body. The flash was not as fortunate as the hotshoe foot snapped when it hit the pavement.
Hot Shoe Level
Another item for landscape photographers that helps keep the horizon level. Although you can fix a sloping horizontal line in Photoshop, you may as well do it right the first time. I just don't understand why these things are so darn expensive. I use the Hama double-bubble.
Flash Diffusers
Portrait and wedding photographers already know this, but for preventing that bright, washed out look on your subject, a flash diffuser is nice to have. I've used the Stofen Omnibounce, and recently picked up the LumiQuest Pocket Bouncer and 80/20, along with Gary Fong's Lightsphere II. I haven't shot enough with them to recommend one over the other yet, but I do see the difference they make in portrait photography.