For backing up files (not operating systems) I use SynkronPortable when running under Windows, and Synkron when running under Linux. It's actually a file/folder syncronization tool not a backup tool though. It's FREE! I use it to backup several 100GB of data to my backups. And yes, with the right settings it can only backup changed files/folders. My only complaints about it would be:
1. If you enable deleting files from the backup that no longer exist in the source, Synkron has a nasty habbit of saving those "deleted" files to its folder. This can be useful, but it would be nice to be able to turn that off.
2. Once you run the sync the program window becomes unresponsive, it isn't frozen, its just working hard on the task at hand. If you start messing with it though, it will freeze the program.