Voice over IP, which these days is more SIP than H.323 and responsible for more unrealistic business plans than the whole internet bubble:
Resources:
- Asterisk - a full VoIP PABX solution in software.
- astmasters.net - a new focal point for the Mac OS X Asterisk community.
- There is also a very nice Mac OS X port with a Cocoa front-end and an active Mac mailing-list
- Jon's Phone Tool can also dial via Asterisk.
- Davi's Page on Free World Dialup
- You Don't Know Jack About VoIP - a nice historical overview
Providers:
- Free World Dialup
- StanaPhone - complete with fax support
- SIPGate
- Wengo
- PhoneSystems