It's all over the blogosphere already, so on the off-chance you haven't seen it yet, I'll link to the best wrap-up I've seen so far: Dare's.
To skip the commentary and jump right in: http://toolbar.msn.com/ , and if you're into PDFs (and others), the IFilter Addins are at http://addins.msn.com/
I'm playing with it now on my work machine; there's no x64 version right now.
For those new to the whole Desktop Search thing - the Windows Desktop Search bit indexes your Outlook and Outlook Express email, and documents on your hard drive, so that you can quickly retrieve files based on keyword searches. I mean really quickly. It takes a while to build the initial index, so you might want to leave it running overnight.
The MSN Toolbar for IE part includes a form filler utility and some level of MSN Spaces integration along with the usual search bumf, and I found it to be a compelling IE addon in the beta release.