Cygwin is a free UNIX -like environment for Windows that enables you to natively compile just about any UNIX package you might need (except some things requiring raw device access and a few specific network calls).
I’ve been using it for many years now, and it provides me with rxvt, bash, SSH, mutt, vim (emacs is there too, if that’s your thing), perl and an increasingly useful X server. I can recompile just about anything on it (it’s pretty much the same as porting to a slightly different variant of Linux or BSD), and most of the dozens of packages I use daily are trivial recompiles.
In terms of real compatibility, it knocked the spots off Interix (rebranded Services For Unix until it faded away into obscurity) and any other commercial alternative. It is also very stable (provided you know what you are doing and keep track of core package revisions).
RedHat also used to provide a commercially supported version that seems to have faded into the mists of time.
|Jul 8||Babun||A pre-configured distribution that includes a sane terminal.|
|Dec 6||Gow||A lightweight alternative that seems moderately interesting.|
|Feb 11||apt-cyg||an apt-like dependency tracker and package installer|
|Older||MobaXVT||a portable version that provides basic remote access features (and X).|
|PuTTY patch||to make it act as a Cygwin terminal. If you’re used to PuTTY, it can be very useful indeed. Me, I’ll stick with the native Windows rxvt port.|
|Outwit||an interesting complement that provides command-line access to a few native Windows features.|
|unxutils||native Win32 ports of some common command-line utilities, useful for places where you don’t want (or need) the full Cygwin environment.|