Speex
The Speex project is an attempt to create a free software speech coder, unencumbered by patent restrictions. Speex is licensed under the BSD License and is part of the Xiph.org Foundation (like Vorbis).The Speex coder uses the Ogg bitstream format, and the Speex designers see their project as complementary to the Vorbis general-purpose audio compression project.
In their FAQ, the Ogg project recommends that applications designers looking for a general-purpose speech coder should consider Speex.
Large application base
There's already a large base of application supporting it, from teleconference to stream, p2p, and audio processing application.
Mostly based on DirectDS filter, OpenACM codec (for example netmeeting) on Windows, or OpenH323 on linux (for GnomeMeeting). There're also plugins for Winamp or XMMS players
See plugin & software page on speex.org site for more details.