Clown gobies are good for little tanks. The only downside is they can be fussy eaters, sometimes VERY fussy, so you want to make sure you pick one that doesn't have a concave belly when it's at the shop. It's best to make sure you get one that you observe eating and get some of whatever it's used to eating at the store.
Neon gobies are another lovely little fish if you like things on the small scale of clown gobies, but neons are very easy to keep and are usually good eaters. They're territorial with related species though, so you can only keep one in a tiny tank - but they don't care about other types of fish usually. I used to have a pico on my desk with one and it would come to watch me when I worked, and it would often sit on my hand if I had to put it in the tank for maintenance; they have little sucker fins that let them sit on surfaces at weird angles.
Another cool option for a small tank if you like interesting inverts is a pistol shrimp and goby pair. Shrimp goby pairs require having a reasonable sand bed so they can create a burrow to live in together. Paired up gobies with their shrimp are usually sold together; if you add individuals of the same species purchased separately, it's hit and miss whether they form a relationship.
