Asterisk is an Open Source PBX and telephony toolkit. It is, in a
sense, middleware between Internet and telephony channels on the bottom,
and Internet and telephony applications at the top. However, Asterisk supports
more telephony interfaces than just Internet telephony. Asterisk also has a
vast amount of support for traditional PSTN telephony, as well. For more
information on the project itself, please visit the Asterisk home page at:

http://www.asterisk.org

In addition you'll find lots of information compiled by the Asterisk
community on this Wiki:

http://www.voip-info.org/wiki-Asterisk