From OSXHints comes a very interesting patch for those of you rolling with G4s.  This one allows you to (slowly) run iMovie '09 on your elder hardware.

To patch iMovie '09 so that it will run on a PowerPC G4, Control-click on iMovie and pick Show Package Contents from the pop-up menu. In the new window that appears, navigate into Contents » MacOS. Now you'll need a hex editor such as the free HexEdit; once you have that, edit the file iMovie in the MacOS folder.

Use the File » Go To Address function in HexEdit to go to the following addresses, and replace each existing entry at those addresses (7C 08 02 A6) with 4E 80 00 20. The addresses are 15fb9c, 15fc7c, and 15fe00.

What you're doing here is replacing the PowerPC instruction for mflr r0, which basically initializes the stack for the called function, with blr, which essentially causes the called function to always return, voiding the purpose of the function. So when the function is the PowerPC check, you've essentially patched your way around it.

When done, save the file and quit HexEdit, and iMovie '09 should launch on your G4.