There's lots of flashy and useful stuff in OSX 10.5 (aka Leopard) which received a lot of attention when it was first released. After several months using a lot of these (time machine in particular is a great addition), I've stumbled onto what I think is my favorite, and the single most useful new thing, and I don't even know for sure if it's new, or if it was ever mentioned in the release hype. It's this:

The finder now sports shortcuts for a time-based view of files you've used somehow. If you're like me and use a lot of apps all the time, and download a lot of stuff all the time, and muck with a lot of different files all the time, the "Search for Today" shortcut (highlighted above) is dreamy. It's like a Lifestream but without the failed proprietary windows-only app: it simply gives you the fastest way to get to "that thing i just used/downloaded/moved/executed/whatever."

A finder window dedicated to keeping this view open is now a fixture on my desktop, and I use it constantly. Wonderful!