I used to do this when I ran windows. I used a freeware program called "syncback" to synchronize my computer to a network drive every night at 4am. It was sort of a "poor man's time machine". There is also a freebie available from Microsoft called "Sync toy".
On OS X, the free sync utility requires a visit to Terminal. I'm sure there's a graphical OS X freeware program that does this same thing. I've probably even downloaded it but never got around to installing it. Of course there is the built in command line sync utility
rsync. There is a more in-depth rsync tutorial
here.
While searching for freeware today, I ran across a number of commercial and shareware OS X file and folder sync programs. I bet that some of them simply put up a friendly graphical wrapper around rsync.
edit: Ok I found a freeware graphical rsync wrapper over at apple.com: It's called
backuplist+.
I also found a shareware program that costs 12 bucks called
nSync.
Please note that I have not tried either of these so be sure to test them thoroughly before you rely on them. I should also mention that I use Time Machine/Time Capsule to handle my sync needs. I browse to the sparsebundle file and grab copies of any files I'm looking for using Finder rather than bothering with the whole animated Time Machine interface. For some reason (perhaps to sell more Time Capsules), Apple is reluctant to support using Time Machine with just any old NAS but I seem to remember they finally backed down and decided to allow Time Machine to work with a usb hard drive attached to an Airport Extreme.