It is available freely (free as in beer), but it's not an open source language (free as in freedom), which is the free I was referring to. Sorry about that ambiguity.
Actually C# (the language) is libre, it's an EMCA standard. The default implementation and IDE(the MS runtime & VS) is not libre, it's gratis, but Mono is fully free software (libre) and nearly fully compatible (both source and bytecode) with the MS runtime and will run on Windows, OS X and Linux.
I wasn't familiar with the language spec being open, thanks for pointing that out.