You could keep...
1 betta (either male OR female), or
2 african dwarf frogs, or
2-4 ghost shrimp
Most other small fish (such as tetras, white clouds, danios, and livebearers) should be kept in larger groups and need more swimming room than a one gallon tank would provide.
If you want plants...
I take it your filter is an undergravel one? If so, keeping plants will be a bit harder but it can be done. Anubias var. nana and java fern would be perfect for that size tank, especially because you'll have low-lighting. Both of these should be tied down to a piece of driftwood, because they will rot if their roots are covered. This can be done with fishing line. Java moss can be trained to cover the gravel of the tank... you just need to weight it down until it can anchor itself. Small varieties of cryptocoryne can be placed in tiny flower pots (if you place them directly in the gravel, the undergravel filter may slow their growth).
D
