Ball pythons are great, as Nate says. And if you get a male, you can be fairly certain it'll stay on the small side...although it will eventually want something bigger than a 15gal tank. Not for awhile though, if you get a baby.

You could go for various colubrids as well. Corn snakes can live in a 15-20gal as a mature snake...they also don't need a lot of fussing with heat. Some, but not as much as ball pythons.

I have a Cape Gophersnake...he lives happily in a 20gal and we just add a little extra warmth with a red bulb when the house gets extra chilly. No fussing with t-stats or humidity. Very easy little guy to take care of. The Capes stay smaller than most of their other Gophersnake cousins.