Today’s FOSS OTD is Google Desktop. Google Desktop is not open source, but it is free, and it is very, very useful. I use it all the time to locate e-mails I can’t seem to find on my own, or to figure out what webpage it was that had some useful piece of information, or even to retrieve cached copies of files that have been inadvertently damaged or lost.
The only real question is whether or not you’re comfortable with the knowledge that Google could use this software to learn about your browsing habits and e-mail. Of course, I seriously doubt they care, on an individual level, but I’d be rather surprised if they weren’t using it on an larger scale to learn which pages are most popular, etc. The risks are basically the same as those of the Google Toolbar.