Cygwin

Get that Linux feeling - on Windows [source]

This is the home of the Cygwin project

What...

...is it?

Cygwin is:

• a large collection of GNU and Open Source tools which provide functionality similar to a Linux distribution on Windows.
• a DLL (cygwin1.dll) which provides substantial POSIX API functionality.

...isn't it?

Cygwin is not:

• a way to run native Linux apps on Windows. You must rebuild your application from source if you want it to run on Windows.
• a way to magically make native Windows apps aware of UNIX® functionality like signals, ptys, etc. Again, you need to build your apps from source if you want to take advantage of Cygwin functionality.