Google Desktop Search is a new tool that allows you to search email (Outlook, Outlook Express), AOL Instant Messenger, Internet Explorer history (unfortunately not Firefox, but they may add it as it is mentioned in their help center) and documents on your computer (Text, Word, Excel, PowerPoint). Interesting how they can do that using just a 400k download. You can still search documents using Firefox though.
Hope they add support for different products in the future (Firefox for web pages, Thunderbird for Email).
If Microsoft did a tool as well, I wonder if search would just be limited to Microsoft products (while everyone else searches Microsoft and non-Microsoft software/documents)? If so, they may not dominate desktop search.