PluralEyes is a tool that automatically synchronizes multiple audio (and their attached video) clips. The cool thing about it is that you don't need to use clappers or time code; it "looks" at the audio tracks and figures out how to match them up. If you're doing multi-camera (or dual-sound) recording, then this could really be a god-send.
It's already available for Final Cut Pro and Sony Vegas, and now they have announced support for Adobe Premiere CS4 & 5.
PluralEyes for Adobe Premiere Pro CS4 and CS5 will be available in beta in April with final release in Q2.
Apple Final Cut Pro and Sony Vegas Pro is available now for $149 USD and users can also download a fully functional 30-day free trial version.