First of all, nothing can really go in a 1 gallon. Goldfish (like stated above) need 10 gallons just for one! The common goldfish grows up to 8+ inches. Bettas would be your only option, or maybe and african dwarf frog? I love african dwarf frogs (not ot be confused with african CLAWED frogs), and you could put maybe 2 of them in that size of tank. They are sensitive to temperature changes, so make sure your heater stays between 75-78*F.
While I am all for the bigger-the-better, I disagree that its cruel to keep a betta in a 1 gallon. I keep my breeding males in 1 gallons for conditioning, and my females in 1/2 gallons for conditioning. It all depends on cleanliness of the water. You go tell any betta breeder that a 1 gallon is cruel, and they will laugh so hard. Most of them have drip systems that are constantly being filled with clean water and drained of dirty water.
Now I know you meant that 1 gallon bowls that are not cleaned often are cruel for bettas to be put in. And I agree. I always recommend people buy at least a 2.5-5 gallon for their betta. But I would not generalize and say that its cruel to keep them in other sizes. The MINIMUM for a betta in my opinion is a 1/2 gallon, but I do get disappointed when people who don't have a mass number of bettas (like myself, and other betta beeders and hobbyists) keep them in that small of tanks.
Alexa