mmap-go is a portable mmap package for the Go programming language. It has been tested on Linux (386, amd64), OS X, and Windows (386). It should also work on other Unix-like platforms, but hasn't been tested with them. I'm interested to hear about the results.
I haven't been able to add more features without adding significant complexity, so mmap-go doesn't support mprotect, mincore, and maybe a few other things. If you're running on a Unix-like platform and need some of these features, I suggest Gustavo Niemeyer's gommap.