I keep my betta (his name is Alpha, did I mention that I test software releases for a living) in a 5 gallon acryllic tank at my office. He's sharing it with two ghost shrimp and an oto catfish. I got the tank at Petsmart a while ago on sale for $35 with a lighted hood and filter that's made for the tank. Works great and is pretty compact.
^ that may be the one I just took him out of, but I think I had the 3g version. I have to remove the clay pots because they're tearing up his fins. I was hoping to put him in a community tank with dwarf or panda cories zebra nerite snails and some shrimp.... not sure if he's the community type though, he's very um assertive