Dropbox: It's free, the basic account can more than hold a semester's worth of work—short of a film school project—and it syncs to all your computers and to the web. "I accidentally deleted my homework" wasn't a very good excuse ten years ago and it's an unforgivable one now. You can sync your passwords, your OneNote notebooks, and access your favorite portable apps from anywhere.
Online Backup: While Dropbox is great for syncing files, if you want to go all out you'll definitely want to check out some full-fledged computer backup tools like Mozy and Carbonite. Check out our Hive Five on best Windows backup tools to get more information.