That tank could house a nice little pacman frog for a while. Eventually you'd probably have to upgrade to a ten or twenty gallon, but they are quite non-motile and are quite content in small tanks for a while. I had one for a while. They are really fun and, lets face it, a frog that can eat mice is pretty cool.
I'm still new to geckos, but maybe viper geckos? I always wanted some of them, they're so nice looking IMO.
And, that is if you wanted to filter the tank for an aquatic species, you could do small fish (very small, i find it hard to keep fish in smaller tanks), dwarf clawed frogs, firebellied toads, firebellied newts, etc. I've gotten out of all but the hardiest of aquatic- too much upkeep for me. Not to mention expense, ride in humidity, etc. I've always had a soft spot for fiddler crabs, also. I had a few while back, and they were really fun to keep.
And still yet, you could probably house a small serpent, such as a corn snake or a milk snake, for some time in there. I've always loved tho looks of the pueblan milk snakes as well as all the color varieties of the corn snakes and even king snakes. Other than that, there is not much else you could really keep in there. I have a five gallon, and at the moment I use it for small crested geckos until they grow as well as a feeding tank for my Pixie Frog. I find ten gallons is a much easier size to work with.
TS
