while not complete, i spent a fair bit of time interested in all kinds of small fish, under 2", looking for even smaller for a rather specific purpose
to build a small self-sustaining tank
i wanted small fish (easy enough) that were omnivores (considerably more difficult for small fish) with a preference towards herbivore diet (suddenly small fish that fit this diet became impossible to find, most being 4" or larger, 6" for full herbivore)
it still did give me lots of time to look up all the pretty small fish
mostly tetras fit the bill, danio's are there too (some, the celestial pearl) i don't remember much now, but green neon at i think, 3/4", cardinal tetra slightly larger, ... the ones i recommended, ...
smaller fish are nicer for a smaller tank as you can fit more in there, instead of having a lone fish (all by his lonesome) you can have a school of fish going to watch ... from other information i have come across, ... more small fish also gives the illusion that your tank is larger than it really is, which is good
there are some fish that are under 1cm, great :), i've never seen any distributor for any of these, the smallest wants a ph of 5 i think, ... (same pH as your can of coke, and you know what's rumored to do to nails but that's all curiosities and possibilities, not realities.
unless some site has come up recently, i have not come across anywhere that lists fish according to size, which would be really nice if you want something special and unique and have a small tank, ... i'm not a web site designer, and have other interests, so i'll wait for someone to come up with such a site, then fall in love :)