You could also try a few tetras- but be aware that they can be nippy. If you do, try and get a betta which has been kept in a tetra tank and looks healthy, that way you know they'll get on. Also look at an African Dwarf Frog, they are completly aquatic and great fun to watch.
Tetras need to be in groups of 6 or more and that would be too many for an 8-gallon tank. I second the pygmy cory recommendation. Just be sure your tank is established. They like it that way!
I tried to add some Pygmy's with my Betta, I removed my Betta first, added the Pygmy's but my Betta still thought they were his dinner
The tank mates he has now is 2 Nirite snails. They are lovely and are keeping the tank clean.
Maybe try one cory first! Just a quick recommendation. . .if you do get pygmy cories, be sure to look at them closely after the sales person puts them in the bag. As I was walking up to pay for mine, I noticed one was dying in the bag (crumpled in the corner). I took the bag back and the fish employee took that fish out and put it (nearly dead) right back in the tank! Then she gave me another one. Well, it wasn't till I got home that I discovered another had a seriously torn fin (which I cannot get to heal). . .Since the fish are SO tiny and move so fast, they can easily be damaged when roughly handled by sales people.