ArsTechnica reviews Parallels Desktop. I didn't even know about the Parallels Tools for Windows XP that enables clipboard sharing and seamless mouse transitions, among other things. And the Bonjour USB printer trick they mention is essential, too.

Parallels costs $50 until July 15, when the price jumps to $80, possibly due to public endorsement from Apple and statements by Apple employees that virtualization will not be a feature of Mac OS X 10.5 Leopard. And it's a kick-ass underpriced product--though you also have to take into account the cost of Windows XP, if that's what you're running.