Apple's iMac went through several design permutations before the company settled on the current "all-in-one computer and display on aluminum arm" configuration. One of the most iconic designs was that of the iMac G4, which held the electronics of the device -- including an optical drive -- in a desktop half-sphere with a 15-, 17- or 20-inch LCD panel suspended on an arm. Modder "Dremel Junkie" (Jonathan Berg) has been bringing classic iMac G4s up to date with new hardware for a while, and his latest work brings a 20-inch iMac G4 into the Mountain Lion era with an Intel Ivy Bridge CPU.
The system uses an Intel NUC (Next Unit of Computing) system neatly implanted into the heart of the iMac G4. It's not just as simple as plugging the Intel NUC into the base of the iMac; Dremel Junkie goes through detailed descriptions of all of the steps taken to make sure that this device remains true to the exterior lines of the iMac G4 while providing the latest in high speed I/O.
Of course, since the Intel hardware isn't "official Apple," the device is incapable of running Mountain Lion without resorting to a Hackintosh solution. Dremel Junkie used an off-the-shelf copy of Mountain Lion and tonymac's Unibeast/Multibeast install method to get OS X running smoothly.
If you have an old iMac G4 sitting around collecting dust and you're handy with a soldering iron and Dremel Moto-Tool, this may be a fun winter project to work on. Two videos are embedded below (about 30 minutes total) that explain the project.