One of my favorite Mac OS X utilities is [Fresh, by Ironic Software](http://www.ironicsoftware.com/fresh/). It's a $9 program that tracks all of the most recently touched files on your Mac and allows them to be accessed via a keyboard shortcut. It also features a "Cooler" area where you can drag frequently-accessed files and folders. I put files and project folders I'm currently working on in the Cooler and remove them when projects end.
Recently, Fresh would occasionally start using a lot of CPU, requiring me to quit and restart the app to fix the problem. I wrote to Ironic support and received an immediate reply from Tom with a link to a patched dev version. 2 weeks later (today), Tom followed up with an official link to the new version. Kudos to Ironic for their excellent support!
If you're into maximizing productivity on your Mac, I highly recommend [Fresh]. It's free to try.