The ability to use several virtual machines to test and learn new operating systems, softwares and test scenarios – amazing, and cheap!
VMWare v6 is in beta. I have it installed and it does offer some improvements, especially for 64bit and Vista and for Dual Core CPUs (all of you with D620s :)).
But the most interesting addition I can see is the new scripting language they've added. All the machines are now fully scriptable with a C like language. I can think of several such usages we can utilize.
If you wish to learn more: Release Notes or Download.
BEWARE! It uninstalls V5.5 - but does not harm or change the VMs themselves.