From aa-asterisk.org.uk wiki
FreeSWITCH is a competitor product to Asterisk, written by some ex-contributors to Asterisk who wanted to do things a different way. GaryH and possibly a few others are trying it out to see if it's any good. So far, its main benefits seem to be that it uses a properly RFC 3261 compliant SIP stack, supports IPv6 and seems to be better written behind the scenes. On the downside, it is a lot harder to configure for people new to software PBXes.