Wine is an Open Source implementation of the Windows API on top of X and Unix.
Think of Wine as a Windows compatibility layer. Wine does not require Microsoft Windows, as it is a completely alternative implementation consisting of 100% Microsoft-free code, but it can optionally use native system DLLs if they are available. Wine provides both a development toolkit (Winelib) for porting Windows sources to Unix and a program loader, allowing many unmodified Windows binaries to run on x86-based Unixes, including Linux, FreeBSD, and Solaris.
Definitly a must for Linux users that have various apps that requires Windows (Word, Photoshop..), i myself use this tool everytime when im on Linux and i must say that its an awesome tool! In some distro’s this is standard included, ifnot you can download it from theyre website (http://www.winehq.com/site/download).