I've been a huge fan of Asterisk, the open source PBX. They have released version 1.4 and a new GUI will be released for it shortly. I run Asterisk as Home using a combination of Vonage for long distance and a local POTS line for local service. I've been very happy with it and have learned a lot of things about Telco stuff and POTS line.
Here is a link to an announcement from InternetNews.com.