Vimproc (License: MIT) by Shougo is an asynchronous process manager. It's a fork of Yukihiro Nakadaira's proc.vim, and requires compilation to run.
Once it's installed, you can run
:VimProcBang to run a command and echo the results. You can also run
:VimProcRead to paste the output of a command in the current buffer.
It also includes functions like
vimproc#system, so you could use it to build other plugins.
It takes a bit of work to get it running because it requires a compiled library, but it should work on Windows, Mac OS X, Linux, Cygwin, and Solaris.
It also includes some colour conversion commands, like