You have Wikia which uses the same software as Wikipedia. It's a complete and total latex support (i.e. I doubt you can add and install new packages) but you have a pretty good support for formulae, just like in Wikipedia.
I think that if you want something that has complete latex support (i.e. you can upload and compile .tex source files) you'd have to hack something of your own.
For further comparison: Comparison of wiki farms