Skype
It’s terrific. I’ve had lots of difficulties getting Apple’s iChat A/V to work across firewalls etc., but Gerard and I had a Skype-powered UK-Netherlands conversation the other night which was flawless, and Quentin and I used it to today to talk through (and solve) a problem with Userland Radio’s flaky FTP uploading. (Well, Q did the solving; I did the talking.) He also lodged some money with Skype and called me on my mobile phone. This is VoIP done right. Skype could be Big.