Today only, CrossOver is available for free download.
For those who don't know, it's a (normally $40) piece of software that lets you run Windows applications, without modification, on a Macintosh or Linux computer, without running Windows.
There are a variety of ways to run Windows on a Mac--you can boot directly into Windows using Boot Camp, or you can run it in a "virtual machine" using VMWare or Parallels. But to do those things, you have to buy a copy of the Microsoft Windows operating system and run it.
CrossOver, by contrast, lets many applications that were written to run under Windows run in other (UNIX-based) operating systems, without using Windows at all.
CrossOver is based on Wine, which is free (and mighty cool). But Wine is kind of intimidating to set up for regular users, especially on the Mac.
I haven't yet actually used CrossOver, but I gather that it's friendlier and easier to set up than plain Wine. I've heard a lot of good things about it; just never got around to installing it 'til now. But now that it's free, hey, why not?
There are actually two free versions of CrossOver today for each platform: the regular version (more stable, better for general use) and the 3D-graphical-games-focused version (may have more features, but is updated more often and thus may be a little less stable). I've grabbed both of them.
For more info about why the software is free today, see an article at The Unofficial Apple Weblog.