I've been using various versions of this plugin off and on for over a year. I don't do much video encoding, but this implementation seems to give very nice results.

My digital camera outputs lightly compressed MJPEG 640x480 video at 30fps, and this plugin can cut the file size by 7 times with very little quality loss. There are a lot of fun features and settings to play around with in order to get the quality and compatibility that you want. You can even turn on diagnostic output to the Console that will let you see the inner workings of the codec and which gives you an estimate of the quality (SSIM, which is more appropriate than PSNR). With newer versions of QuickTime (above 7.5, I believe) you can turn on all of the features, including the high profile h.264 features, and it'll still play natively. For older versions of QT, you'll either need to lessen the features or get Perian (a good choice anyway).

It's good stuff: updated often, can take advantage of as many cores as you can throw at it, and produces higher quality and more configurable output than QT's built-in h.264 implementation. I don't know why people still use DivX!

PS There's not much built-in documentation about what all of the options mean. You can go here for a summary of most of the current options: http://mulder.dummwiedeutsch.de/pub/x264/. It's written for a piece of Windows software, but most of it is still applicable.