I've not used it, but I've heard people say good things about SyncToy, free from Microsoft:
It's a simple tool to keep folders and drives synchronized. If you're on Windows, check it out.
Personally I use SyncBack, which isn't free but does a fantastic job of keeping my crap backed up to external drives. After it's run the first time, it only copies new and changed files. And it doesn't use any sort of proprietary database for storage, it just copies the files over so they're completely accessible.