A bear canister is an important piece of gear to have if you find yourself in bear country. Outside of a few parks you don't actually need a bear canister, and there are alternatives to bear canisters, which you can find below, but a bear canister does provide a certain peace of mind that your food will still be there in the morning. There are some big differences between bear canisters, however, and a bear canister comparison is a useful thing to look at before buying.
Before you Buy a Bear Canister...
Figure out where you are going to be using your bear canister. A lot of people end up buying bear canisters because they are required in many national parks where bears are numerous. Before you buy, be sure that your bear canister is approved for use in those parks. This will save you from having to buy a second, approved bear canister later. You can find the approved bear canisters for Olympic Park here, and for the Sierra region here. Keep in mind that canisters may be added or removed from the lists, depending on the reports that the Parks Service gets.
Bear Canister Comparison
There aren't too many bear canisters on the market, and these are the best of them.
Garcia Bear-Resistant Container - This bear canister from Garcia is a good all around bear canister. It holds 10 liters, which is 614 cu in, or put another way, about enough food for one person for 6 to 7 days. It weighs 2.75 pounds, or 1.24 kilograms. It doesn't have the biggest opening, so you might have a little trouble getting larger food items in. It's also black, which is a problem because bears often drag bear canisters away in to the bush, and finding a black canister wont be the easiest thing in the world to do. It is approved by the Parks Service as of this writing.
Bear Vault BV500 - The BV500 is an excellent bear canister. It's relatively lightweight, at 2.55 pounds, or 1.16 kilograms, and holds quite a bit too, at 11.5 liters, or 700 cu in, which enough room for about a weeks worth of food. It also has a big, wide opening and is clear, which wont help you find it after a bear drags it away, but will help you find that Snickers bar at the bottom of the bear vault. It won the 2004 Backpacker Magazine Editor's Choice award, and is approved by the Parks Service as of this writing. It also comes in a smaller size, the BV450.
Counter Assault Bear Keg - The bear keg is heavier than the other two, at 4.2 pounds, or 1.92 kilograms, and does not hold significantly more, at 11.7 liters, or 716 cu in. The one thing that it definitely does have going for it is that it is bright yellow, so if a bear ever drags your Bear Keg away in to the bush, you should have no difficultly finding it again. But the significantly extra weight is definitely a big drawback.
Bear Canister Alternatives
If you're just camping in bear country, and not in a park where a bear canister is required, then there are some lightweight alternatives to a bear canister, such as hanging your food. The PCT method is one popular hanging method, named after the Pacific Crest Trail, which travels along the Rockies from Mexico to Canada, and through plenty of bear country. All you need is a stuff sack for you food, a carabiner for the stuff sack, and some lightweight cord to get everything hanging from a tree.