I agree with other responses. If you really like the look and idea of a pleco for what they are (they are really cool, almost prehistoric looking animals), then you can get a bristlenose or clown pleco, those stay relatively small compared to a common pleco.
Make absolutely sure you don't let the people at petsmart talk you into getting a common pleco. They grow up around 12 inches long, which is much too large for a 20 gallon and they produce TONS of poo... one got sent to me once in a used 55 gallon aquarium and I had to take him to my local fish store and see if they would take him, because he was actually too large for the tank and was leaving garland-like strands of poo all over everything.
That being said, I do own a bristlenose, and she still produces a lot of waste, but I like her, and they can live quite a long time when cared for properly.
Corydoras, as suggested by ImACoolguy, are very cool little fish too. They need to be kept in small groups (of 4 or more I believe), but they are much smaller than the bristlenose pleco and will probably be out and about more moving around on the bottom of the tank.