oRTP - a Real-time Transport Protocol (RFC3550) stack under LGPL
Features:
* Written in C, works under Linux (and probably any Unix) and Windows.
* Implement the RFC3550 (RTP) with a easy to use API with high and low level
  access.
* Includes support for multiples profiles, AV profile (RFC3551) being the
  one by default.
* Includes a packet scheduler for to send and recv packet "on time",
  according to their timestamp. Scheduling is optional, rtp sessions can
  remain not scheduled.
* Supports mutiplexing IO, so that hundreds of RTP sessions can be scheduled
  by a single thread.
* Features an adaptive jitter algorithm for a receiver to adapt to the
  clockrate of the sender.
* Supports part of RFC2833 for telephone events over RTP.
* The API is well documented using gtk-doc.
* Licensed under the Lesser Gnu Public License.
* RTCP messages sent periodically since 0.7.0 (compound packet including
  sender report or receiver report + SDES)
* Includes an API to parse incoming RTCP packets.