Manga/Anime and asian cultures as a whole. Those were basically their greatest influence.
The visuals are heavily based on manga and anime: clean and slated, with a futuristic feel.
The fluff itself and part of the design choices are clearly oriental-themed. The Fire Warrior armour is remarkably similar to a Samurai's armour, for example. Water Caste members use funny hats similar to those conical hats worn by asian peasants. Their entire society and philosophy of devotion, selflessness (and segregation) is also very inspired in the old asian societies, specially the japanese.
Most other things are just staple from science-fiction. Plasma-launching weapons, neurally-controlled battle armour, utopian society, things like that.
Quite possibly, the only original thing in the Tau is their visuals. And maybe not even that, because they really look like the Protoss.