Smart Folders are easily the hottest new item in Tiger, and they're powered by Spotlight's index which just makes them even more powerful. The two places I've found that I use them the most are in Mail and the Finder, so I'll just run over the best applications I've found.
The one that leapt out at me, since I tend to lose downloaded files even though I store them all in a file called Downloads in my Documents folder:
Recent Documents - Quickly organize your Documents and Desktop folders with a simple Saved Search that checks for items in Documents created or modified since yesterday. Works best with downloads from a program where you forgot where it was downloading to...
::plot further evile while I await the opening of the nearby Apple Store::