The Open IMS Core is an Open Source implementation of IMS Call Session Control Functions (CSCFs) and a lightweight Home Subscriber Server (HSS), which together form the core elements of all IMS/NGN architectures as specified today within 3GPP, 3GPP2, ETSI TISPAN and the PacketCable intiative. The four components are all based upon Open Source software(e.g. the SIP Express Router (SER) or MySQL). For almost three years the Open IMS Core has formed the heart of the Open IMS Playground @ FOKUS
In addition to a short overview of the Open IMS Core Components you can find on these websites also some more information on the background of the project.