Benjamin Kowarsch emailed me to give me the thumbs up on mentioning this. Ben's company, Sunrise Tel, have been putting together graphical front ends and configuration wizards for the Asterisk IP-PBX running on Mac OS X.
There is now a placeholder website at Astmasters.net -- "where the Macintosh Asterisk community meets". Of particular interest, if you request an account on the Macintosh Asterisk Mailing List, you can get a free VoIP account:
Regular members of the Macintosh Asterisk Mailing List may apply for a free account on the Astmasters VoIP service. This service supports both SIP and IAX and is predominantly intended for Macintosh Asterisk users to test connectivity and to be able to call other list members free of charge. However, it is also possible to make and receive calls to and from other networks.
Account holders will be provided with an Astmasters.net SIP URI which can be called from any SIP compliant IP phone. Regular PSTN telephone numbers can be called, provided that the numbers are E164 directory listed. Both VoIP calls and VoIP to PSTN calls are free of charge. User controlled Do-not-disturb (DND) is provided by default and incoming voicemail will be sent by email.Astmasters VoIP Service
Ben is going to be completing peering with Free World Dialup as well. Oh, if only I had a few weeks to focus on nothing but Mac VoIP for a while...
Unrelated from Asterisk on Mac, I just found out about Jon's Phone Tool, an immensely scriptable application that lets you do all sorts of phone-related things from your Mac. Including, dial via Skype from your Address Book. Check it out.
Skype dialing is described in the user guide (select the "Skype" link from the left hand menu).