Asterisk is an Free/Open Source Virtual IP PBX and VOIP server which supports lots of VOIP protocols like SIP,H323,IAX,IAX 2 and more protocols also it support using analog and digital likes like T1/E1 via using special hardware produced by many vendors including the sponsor for the Asterisk project Digium which founded by Mark Spencer the original author of Asterisk.

Also Asterisk considers a very powerful and very cheap PBX system, since it support amazing features like call recording,IVR menus,call transfer, call picking up, park calls, music on hold, passing caller ID,caller ID name look up" in case your PSTN is a SIP provider and supports Name ID " or you can use it locally by creating a lookup database, blocked numbers, voice conference, video , text chat and it supports having SQL databases as a backend for configurations and CDR. This is not everything. Also it can be the only PBX you use, you can integrate asterisk to any analog PBX system.

1. Free/Open source.
2. Supports many VOIP protocols.
3. Very powerful PBX.
4. Cheap since it's Free as free of charge.
5. Can work as a FAX machine with fax/voice detection, Fax to mail Gateway.
6. Video and text chat.
7. Audio Conferencing.
8. Voicemail , and can be attached to the email.
9. much more.

1. Not easy to configure.
2. Most of GUIs dose not support all the configurations configurations.
3. AFAIK dose not support Video conferencing till now "1.2.x stable release"

it's a full package, very computable with each application within.

1. There is another alternatives, like openSER which works only as a SIP server , no IAX supports as far as i know and not a PBX.
2. There is also openPBX which is a fork of Asterisk due to some license issue related to that Asterisk is a dual license " GPL, Digium License] AFAIK, but i didn't give both a real try and time.

Give it a try or u can get Asterisk@home " TrixBox " and give it a try..