SUA, formerly called INTERIX, is now 100% POSIX compatible. There are a lot of ported apps for it. Even whole Linux distros like Debian have been ported. SUA/Interix comes with Windows 2003R2 and up (including Win7 which has the latest 6.1). I also successfully compiled BASH without it directly supporting SUA.
Previously only parts of POSIX were implemented. The newest version is fully POSIX compliant and is (iirc) mapped more directly onto the Windows kernel than before, causing an increase in speed.
Beware however that I (on a personal note) experience memory problems when I keep BASH open for several days. The problems seems to affect the whole SUA system. I don’t know the cause of this so it could be something with my setup, but worth testing before considering it for production use.
Links:
Wikipedia article on Interix
Debian interix port
SUA Community with bundles of apps/utils