Linphone is an audio and video Internet phone with GTK+ and console interfaces. It uses the SIP protocol, and is compatible with most SIP clients and gateways. It can use various audio and video codecs such as Speex, GSM, G711, G722, ilbc, amr, Theora, H263-1998, MPEG4, H264, VP8, and snow.
Mediastreamer is a portable C library that allows you to create and run audio and video streams. It is designed for any kind of voice over IP applications. It features RTP connectivity, audio codecs (Speex, iLBC, G711, GSM, and AMR), video codecs (MPEG4, H263, H264, and Theora), sound card I/O, wav file streaming, webcam video capture, echo-cancellation, conferencing, parametric equalization, and various other utilities. It has a modular design that makes it extensible through plugins. This is the media-streaming component of linphone, a GPL SIP video phone.
mptc is a Perl script to help you work out whether you're on the cheapest mobile phone network/tariff for your phoning habits. You feed it an itemised phone bill and tariff pricing information, and it will tell you how much that bill would have cost on different tariffs. So far it only supports UK network pricing systems, but could easily be extended and applied in other countries.
o2-sms.pl is intended for customers of O2 Germany that have an account at www.o2online.de. It can send SMS messages via the WWW gateway. Use the -f option to send a flash SMS (it will pop up on the recipient's display immediately). The message text can have up to 780 characters. SMS to wired recipients are limited to 160 chars, as they are being sent as "text-to-speech".
oRTP is a library implementing the Real-time Transport Protocol (RFC3550), written in C. It is easy to use and provides a packet scheduler for sending and receiving packets on time, adaptive jitter compensation, automatic sending of RTCP compound packets, and the RTCP parser API. It works with IPv6.