Backups with Time Machine to a network fileserver
Time Machine is the best thing about Leopard. But I recently became annoyed that I had to plug in an external USB drive to use it. I have a file server running Ubuntu in the bedroom under a couch that has all of my photos and music and valuable data on a RAID array, so why not put my laptop backups there, too? This turns out to be a bit more complex than expected:

1. Install netatalk and avahi on the Ubuntu box. apt-get is your friend.
2. Set up an AFP share so that you can view it in the Finder. Use AFP, not SMB.
3. Open a terminal and enter:

defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

4. Open the Time Machine preferences and change your backup disk to your AFP share.

If you've done this correctly, you'll get an error when Time Machine starts: "The backup disk image could not be created." So, now for the fun part:

5. Fire up Disk Utility and create a new disk image on your AFP share called "ComputerName_MACAddress.sparsebundle.dmg", substituting your own computer name and MAC address (without the colons, of course). Make the disk image large enough to hold backups (in my case, the bare minimum size was 36 GB--your mileage may vary). For "Partitions", choose "No partition map". Creating the disk image takes several hours.

6. Once your disk image has been created, start a Time Machine backup and it should "just work" (ha ha).

I am a tad disappointed that I had to do this much hacking to get my backups working over our wifi but now that it does work, I'm quite happy with the setup (and it sure beats paying $275+ for a Time Capsule!).