If you're on windows, you should check out "Volume Shadow Copy Service":

They allow the creation of consistent backups of a volume, ensuring that the contents cannot change while the backup is being made, avoiding problems with file locking; because the backup software is using a read-only copy of the volume, it is able to access every file without interfering with other programs writing to those same files.

en.wikipedia.org/wiki/Previous_Versions

Note that this is used in MS's own backup utility, which many consider crap, but some people tell me the command-line version is actually good for something.

Just thought I'd offer this bit of information, which many people don't seem to know. Other than that I was also going to suggest Bacula, like another poster farther down already has.

Johannes