VoIP - SIP and RTP stacks, softphones, user agents, STUN

RAT - Robust Audio Tool; Supports a large number of codecs, ... License: Free; Homepage: http://www-mice.cs.ucl.ac.uk/multimedia/software/rat/
JMF - Java Media Framework: Can receive and send RTP streams; Homepage: http://java.sun.com/products/java-media/jmf/
MP3/RTP Plugin for Winamp: Homepage: http://www.live.com/multikit/winamp-plugin.html
Vomit - Voice over Missconfigured Internet Telephones: Plays back captured voice conversation; Homepage: http://vomit.xtdnet.nl
RTP Tools: Several RTP utilities from the Columbia University; Homepage: http://www.cs.columbia.edu/IRT/software/rtptools/
UDP Packet Reflector/Forwarder: A tiny tool which forwards or reflects UDP packets. You can also add delay and packet loss. Very useful if you want to test RTP applications. Homepage: http://www.cs.ucl.ac.uk/staff/s.bhatti/teaching/z02/reflector.html.