Opal is a full-featured voip library that supports a variety of protocols and
multi-media fornats. Features include:

    * Low latency RTP stack designed specifically for real-time multimedia
    * Full featured H.323, SIP and IAX2 protocol stacks
    * Audio codecs including G.711, GSM06.10, Speex and iLBC.
    * Video codecs including H.261 and H.263
    * Run-time loadable codec interface for proprietary or codecs such as G.729,
      H.263, H.264 and MPEG4
    * Completely Open Source using the commercially friendly MPL 1.1 license

Through the use of a platform isolation library called PTLIB, Opal can run on
tiny embedded systems or multi-processor servers.

BTW: OPAL is an acronym for Open Phone Abstraction Library, but that is not
really important :)

WWW: http://opalvoip.org/