George Ou does a mini-review, actually more of an analysis of the Asterisk open-source solution on his ZDNet blog that was pretty interesting. He mentions other open-source solutions such as OpenPBX and Pingtel, but the article mostly focuses on Asterisk.

He touches upon IAX (Asterisk proprietary protocol) vs. SIP and why IAX has some technical advantages due to better NAT traversal, but does state in the comments that a separate blog entry is in order to fully compare the two VoIP protocols.

In any event, it's an interesting read you should go check out:
George Ou » VoIP and open source, the next great frontier - ZDNet.com