Mixxx is software for DJs that provides everything needed to create live mixes. It allows you to beatmatch songs and crossfade them together, like you would with turntables and a mixer. It runs without any extra hardware, and offers a wide range of advanced features including comprehensive MIDI controller and vinyl control support. To obtain the lowest latency and highest responsiveness, it uses hardware video acceleration and takes advantage of multi-core CPUs.
crtmpserver it is a high performance streaming server able to stream (live or recorded) in the following technologies: to and from Flash (RTMP, RTMPE, RTMPS, RTMPT, RTMPTE), to and from embedded devices (such as iPhone and Android), from surveillance cameras, and with IP-TV using MPEG-TS and RTSP/RTCP/RTP protocols. Also, crtmpserver can be used as a high performance rendesvous server. For example, it enables you to do audio/video conferencing, online gaming, online collaboration, and simple or complex chat applications.
Theory is a Python/Pylons-based Web client for the MPD system that uses Ajax where it makes sense. It includes drag/drop playlist reordering, Amazon cover art retrieval, a full-screen "party" mode, lyrics fetching from lyricswiki.org, automatic linking to the artist's Wikipedia page, and a decent theme.
Crossfire is a Firefly Media Server client written for the Apple iPhone and the iPod touch. It has support for streaming, browsing, and searching music content. It supports both Mobile Safari and FireFox, but can be configured for various other devices such as Windows Mobile, Symbian, etc.