OS X ships with Finder and Spotlight, but I never use either one. It's all about Path Finder and Quicksilver.
If you're not sold on Quicksilver yet, I'll let some funky Europeans break it down for you:
Path Finder is absurdly good too. You can use it as a plug-in replacement for the Finder - the Finder will still reactivate itself from time to time, or more accurately, various apps may bring it back to life, but if you want to set up Path Finder so that it knocks out the Finder when it loads and just gives you a file browser for grownups with serious Unix power, that's a few checkboxes on the preferences pane to do.
Update: My first few days with Path Finder were mostly Finder-free; recently the Finder's been sneaking back to life, and every time I quit it, it just re-establishes itself. There are slightly more aggressive ways to shut the Finder down, but I'm not experimenting with those until I've actually bought my copy of Path Finder and gotten comfortable with it.