What does SIP stand for? Session Initiation Protocol which is just a protocol that defines a way to share video and audio between computers. This protocol is the next gen protocol to allow for telephone and video phone systems of tomorrow.
Looking for a nice hardware SIP ATA? I've got the Sipura 2100 and it is wonderful. Two fxs lines with T.38 support! Sipura has been aquired by Linksys, so you may have to look for something from them.
Here is the Specs for SPA-2100, SPA-2100 Quickstart Guide, and Firmware downloads.
Here is the asterisk wiki page for the Sipura SIP devices.
Another really great feature of this device is the integrated IVR system. In other words, if it is enabled in the config, you can dial "****" to go into a config IVR where you can check the IP(110), check firmware(150), enabled/disable web server of SPA(7932)
Today I found a free for non-comerical solution.
Kapanga ------ I've yet to use it.... but this looks like a nice client. Includes video and audio processing. More information coming soon!
CounterPath and eyebeam.
Yate - Yet Another Telephony Engine