XPostFacto

XPostFacto (formerly UnsupportedUtilityX) is a shareware program created by Ryan Rempel that allows Mac OS X to be used on pre-G3 Macs. It can also be used to install later versions of Mac OS X on OS X-capable machines that Apple has declared unsupported, such as the early iMacs, beige Power Macintosh G3, the PowerBook G3 "Wallstreet" and "Lombard."

Currently, online retailer Other World Computing is a major supporter and contributor to XPostFacto. XPostFacto is currently in its fourth major revision, which adds support for installing Mac OS X "Tiger."