GeeXboX is a standalone media player Linux distribution, similar to MoviX. It's a small bootable CD that allows you to play your favorite video (DivX, XviD, H.264, MPEG 1/2, VCD, DVD, OggMedia, Windows Media, RealMedia, etc.) and audio (MP3, Audio CD, Ogg Vorbis, FLAC, MPC, etc.) files. It also supports networking, and is able to play media from Windows/Samba share, NFS, UPnP A/V Media Servers, RTP/RTSP servers, or SHOUTcast. It supports TV-out, TV tuners, DVB cards, and WiFi cards. It is based on MPlayer, and can be used on any x86, x86_64, or PowerPC computer. It's easy to modify the source to build your own GeeXboX or use an alternative boot method.
A.M.I.C.U.S. (Automatic Multimedia Installation Configuration Utility System) helps users quickly and easily install and configure MythTV on generic PC hardware. It uses the Debian Netinst CD to install GNU/Linux and just the required packages to allow a functional MythTV on low end hardware.
Source Ruckus Linux is an embeddable, robust Linux meta-distribution. It's a collection of build tools, source packages, scripts, and documentation all ready to be built into a Linux distribution. It can be used out-of-the-box, but its real strength lies in being used as a base for your own distribution. In other words, it exists purely to be forked, although it can be used as-is.