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KallistiOS is a free, BSD license-based development system for the Sega Dreamcast game console. The project was initiated in 2000 by Dan Potter, then garnering a team of free software developers over the Internet. Kallisti, from the Greek "to the fairest"; Kallisti OS (Operating System); or the abbreviation KOS, which is pronounced like "Chaos".
KallistiOS is the successor of libdream, the first free developer library for Dreamcast. Libdream was also written by Dan Potter and published in mid-2000. It served as the basis of some hobby projects, but was soon replaced by KallistiOS.
KallistiOS allowed the emergence of a homebrew developer community on the Dreamcast. Several free and commercial games have been created using it, such as Last Hope (video game), Feet of Fury, and Inhabitants.