Bipods are very popular with the Tacticool crowd- the guys who like to fill every rail on their carbine with as many accessories as they can fit. But they're actually very useful. I recently purchased a Caldwell XLA Pivot bipod for under $37 that delivers solid functionality for not very much money. Here you see it mounted on my Crosman Marauder Mk2 air rifle, but I think I'm going to buy a few more for two of my .22s for range and field use.
If you like the idea of a bipod but want the best you can get, the US military favors Harris bipods, I'm told. They offer even more variety in styles and attachment at somewhat higher cost. The Harris equivalent of the Caldwell XLA is the Harris S-BRM, and it can be had for about $97.