Go here...
POTN Accessories & Storage
...and look for the stickied bag review thread on the top.
Also look for the thread titled like "show me your bag" or something like that. A boatload of info.
Domke, Crumpler are often the top pics. However there are many makes that do well, you just have to find something that works for you.
Think Tank makes really nice bags and the like. The materials aren't super tough like Domke's canvas or ballastic's, but the build quality is nice.
Here is my summary:
Domke: Known for their thick canvas bags. Hardly any zippers (if at all) to be found. Most of their line is shoulder bags, but they come in a huge variety of sizes and depths. They can be over stuffed by adding rings to their fasteners. They will last years, and they get better with age. (too stiff when new). Most of their line has a ballastic nylon alternative.
All have metal hardware, not plastic.
Crumpler: They have a lot of backpacks, but they are known for their shoulder bags that have the messenger bag look. Very hip. Construction is nice, and most models aren't that much more in price than your garden variety Lowepro. Thick partitions and cushioning can be found in there. Some might gripe that you can't over stuff these bags like the Domke's, but they are good.
Lowepro and Tamrac: They offer everything under the sun, and they split their offerings under pro and consumer lines. Shoulder bags, messenger bags, backpacks, belt packs, modular belt systems, sling bags, everything. Build quality is decent, not the best, but it's nothing to scoff at. Higher end ones will have other touches like weather sealed zippers, and some crazy backpacks that can get drenched and still keep the gear dry.
I like Lowepro for their small touches. Even most of their consumer bags will have "AW" attached to their product name. They have a deploy-able cover you can put over it during a downpour.
Tenba: Slightly better build quality at a slightly increase it cost when comparing to Lowepro.
Kata: They have adventurers in mind. Good stuff if you hike a lot.
Think Tank: I can't explain them, just visit their website. Nice specialized stuff. You might want to check out their Urban Disguise line, they come in all sorts of sizes.
I would really look into Crumpler, Domke and Think Tank Urban Disguise. Get something that might be bit bigger than what your needs dictate right now if you plan to buy more lenses or flashes. Like if you have 2 lenses now, get something that just doesn't carry two, get a 3-4 lens sized one. Just don't get too crazy with it.