Do you want an aggressive tank or a peaceful tank?
Although yellow clown gobies are small, colorful, and peaceful, (as are the brown, black, and green ones) -- these guys often come into stores with painfully pinched stomachs and avoid them on the whole. There are green "clown/coral" gobies with red markings on their heads that have the same body shape but tend to grow slightly larger than the solid colored clown gobies, and I have found some robust fish available of this species. I have also had some luck with the larger citron goby, which would be comfortable in a smaller tank.
Beyond these dorsally flattened gobies (and I have housed the green and citron together with no issues), there are the other small peaceful gobies that have been mentioned: the neon cleaners, greenbandeds, redheadeds, Trimma spp., and Stonogobiops hi-finned gobies. Other possible gobies that are small and peaceful include the Randall's shrimp goby (I housed one with the aforementioned green and citron) and the popular yellow shrimp goby.
Other small peaceful fish include the readily available two spot blenny {note that many blennies grow large and can become aggressive}, the yellow assessor (available captive bred and will swim out and about at times), the before mentioned Plectranthias inermis, Tryssogobius colini (which I know nothing about), and the bluestripe pipefish (which may need weaned to frozen food).
Research would have to be done on any cardinal, wrasse, dwarf angel, or damsel that you might consider -- many grow large or are prone to aggression, particularly in small spaces. Two spot and rainfords are known to be prone to starvation (I realize that jayelblock was stating what is in his/her tanks, not necessarily recommending those fish, but I just thought I would put that out there.)