For a small tank like a 5 gal, your best bet would be a single or pair of firebellied newts or toads. Reptiles tend to require more room, but if you are lucky, you may be able to locate a nice worm snake or ring necked snake.
Honestly, I would not go below a 10 unless there is areally good reason you need to use a 5.
The choices Ceph88 gave are all good ones. Another possible you may find in you local pet store is the oriental firebelly toad. They are not very big and are reasonably inexpensive.
go invert and get a tarantula!!! they are sooo easy to care for, u only have to feed em every week or 2. there are soo many different species that are relatively cheap and they are really rewarding pets IMO. i <3 my tarantulas!!
I'm going to just ignore the cham thing..knowing I might end up flaming, I have the perfect suggestion! Viper geckos! Awesome and really cute..they're a pakistan native and are terrestial.. a single adult will fit in a 5 gallon..assuming it's a 5.5 gallon...(Same as 10, but half it's height) Expensive geckos as far as cost $40, but easy to keep, only eat crickets..only need to be sprayed for water (almost like spraying a nepenthes, 'cept not as heavy) and really, really cool