Google Desktop is the coolest thing since... Google, I guess. It's a indexing tool that you run on your computer. It will index (Outlook) email, text files, etc. If you use Internet Explorer, it also watches where you surf, caches web pages for future reference, and so on. It even keeps a history of old version of your files and web pages.
When you do a query on plain old Google.com, the results from your computer are included in the usual Google results. So very, very, cool.
I've only seen screenshots of their email stuff. It's not as cool as Haystack (my prototype of an email search engine), but it's still damn cool.