Mac OS X Leopard only: Back up your Mac with Time Machine not to a local hard drive but to a shared network disk with iTimeMachine. This simple two-button desktop utility makes your shared network drives show up in Time Machine's possible backup destination list. But it isn't as user friendly as it could be.
To set things up, first, mount the network disk onto your Mac. Just browsing Network in Finder won't do it; make sure you hit "Connect to Server" from the Finder Go menu and mount the shared folder as a drive on your Mac. (Here's more on sharing files across the network with Leopard.)
Once you've got a shared drive mounted, download iTimeMachine and install it. When you launch it, you'll notice it's just a dialog with two buttons, as shown.
Press the "Enable Airdisk/Network Disks" button. But don't be surprised when nothing happens. iTimeMachine doesn't give any indication or confirmation that it's done what you asked. The two buttons remain unchanged regardless of which you hit or how many times you hit them. (iTimeMachine developers: make that a single button that toggles.)
Nevertheless, once you've hit the Enable button once, you can quit iTimeMachine and go into Time Machine's settings in System Preferences. From there, hit the "Change Disk" button and your network disk will appear in the list of possible backup disks, as shown.
The first time I tried this out on my MacBook Pro, I got nothin'. It took a few retries and remounts to finally see the network disk in Time Machine's options. So, while iTimeMachine's interface could be smarter and friendlier, it does get the job done. iTimeMachine is a free download for Mac OS X Leopard only.
For more Time Machine tweaking, see also Schedule your Time Machine backups with Time Machine Scheduler. Thanks, John!