Lucene Desktop
Aside: Lucene is super neat but if I wanted to run Java on my desktop, I'd just write a fork bomb and be done with it. Ahem. PyLucene might make a better choice since it's compiled natively.
All of this sort of misses the main selling point of desktop search, which I think even Google has missed; it's not the index, it's the integration. Dashboard hit this nail right on the head, but it had a lot of influence in convincing other Gnome apps to conform to it's interface.
You know what would be neat? If Dashboard could tell me about relevant stuff no matter where I was logged in from. If I could tunnel data through IM to my Dashboard installation then it would know what I was working on, writing, reading, listening to, etc, and give me recommendations and relevant recent data no matter where I'm at.
Wait... the data doesn't have to hosted on my personal computer. This would make a great service if you could make people feel at ease with this vast amount of personal knowledge being churned by somebody else's computer.
# — 13 November, 2004