First up, from the OS4 team, is the brand spanking new update for Amiga OS 4.0. Thought the Amiga was dead? Think again. The new release includes support for USB mass storage devices and a Kernel update, taking it to 51.20. You can find more details on the release at AmigaWorld.
Secondly, if you're fed up of your boring old iPod menu system, why not stick Linux on it? Some crazy chaps have created a distribution which will install onto the device and dual boot, as well as coming with a funky GUI, named Podzilla. The extra functions the OS provides includes on the fly recording and a number of new games. You can find some more details about the project here.
Whilst technically not an operating system hack, it seems appropriate to mention here that a crazy console gamer has worked out a way to overclock his Nintendo (yes, the original) by a whopping 200%. If you're fed up of slow-down in Mike Tyson's Punch out, or crave faster targets in Duck Hunt, you should check out his tutorial here.