Windows/Mac/Linux: Free virtualisation tool VirtualBox, technically Oracle VM VirtualBox now, has quietly added "experimental support for Mac OS X guests" to a beta release. That's good news — at least for Mac owners, and those willing to dig into serious Hackintosh tweaks.
Mac owners looking to try out buggy software or keep entire other OS X installations handy will see the main benefit from having OS X as a guest on their Mac hardware. VirtualBox won't do much to make Apple's operating system work on non-Apple hardware, but from what we've seen around the net, we'd bet VirtualBox's opening of this can of worms will lead to some nifty, Hackintosh-style workarounds that will get a Mac running on Windows/Intel hardware.
Also new in VirtualBox's 3.2 beta is support for deleting machine snapshots while the machine is running, multi-monitor support for guest systems, and quite a few 64-bit fixes and updates noted in the forum post below. If you want to skip right to downloading the binaries, you can do so at this text-riffic download page.
VirtualBox 3.2 beta is a free download for Windows, Mac and Linux systems. If you already know of some neat and, shall we say, unofficial OS X workarounds that make use of VirtualBox, tell us about them in the comments.