A "Hackintosh" is the popular moniker for those Frankensteinian machines that run Mac OS X on standard Intel hardware.
It all started when the 10.4.3-based development platform DVD image was leaked on the Internet, and since then a surprisingly active (if rather elusive) community has sprung up around it and appears to have cobbled up a bunch of patches to bring it somewhat up to date, but with extremely limited hardware support (you're basically limited to what the original Intel-based development platform had and a few extra tidbits - common complaints are lack of audio, Wi-Fi and even video drivers).
Despite the shady aspects of this, there is an interesting side-effect of this activity - there is suddenly a lot more interest in Mac OS X driver development, which might be useful for third-party hardware (i.e., peripherals, USB-to-serial adapters, WWAN cards, etc.).