Free Internet Telephony
AdCalls. If you don't mind looking at an ad banner to make a phone call. It's totally free and can be used from your computer (one down side: you will be listed as "unknown (or) private caller"
Vonage, which Paul and I are going to be signing up for real soon, which includes not only unlimited long distance but also internet telephony. There's a charge for it, but it's reasonable.