Wine, which is, in fact, not an emulator, is a reimplementation of the Windows API on top of X, OpenGL, and UNIX. It is completely free in that it doesn't require any files from Windows, but you can optionally use Windows .dll files. Wine allows you to run unmodified Windows binaries on x86 UNIX-like systems such as Linux, BSD, Mac OS X, and Solaris.
While compatibility is not perfect yet, thousands of applications have been reported to work very well. Check http://appdb.winehq.org to see the details for your favorite applications.
Packages for your flavour of Linux or BSD can be downloaded from the Wine download page, or you can wait until it hits a repository near you.