FreeFileSync is great if you just want to keep an up-to-date copy of everything and you don't need multiple incremental backups. It compares file size and timestamps and only copies files that have changed. You can save batch jobs as a file and click on it whenever you want to sync. You can even make the batch job autorun when you connect your external hard drive.

Throw TrueCrypt into the mix, and you've got a pretty good and secure backup.