Beyond Compare
If you're a software developer, web designer, or simply use files on more than one computer, you need Beyond Compare:
Beyond Compare is a powerful time-saving utility for comparing files and folders on your Windows system.
Use it to manage source code, keep folders in sync, compare program output, and validate copies of your data. Beyond Compare can create a snapshot of a live directory structure, and compare it against the live copy at a later date.
Beyond Compare helps you analyze differences in detail and carefully reconcile them. It commands a wide range of file and text operations, as well as script commands for automating tasks.
I've used BC for years, and I use it almost every day. Not only is it the best tool I've found for synchronizing files from my laptop or desktop PC to various servers, but it's even saved me when I had to replace a bad hard drive and selectively copy the contents of the old drive over to the new one. I can't recommend it highly enough.