- Marratech (commercial system).
- Elluminate (commercial system, cross-platform app sharing!)
- iVocalize (commercial, but inexpensive). It looks like a work-in-progress, but the TLT Group likes it and uses it.
- ePresence (open source, but I'm not sure if it works on Macs)
- and if you only have a few Mac people to talk with, iChat AV is amazing. Using Trillian I think you can even iChat with your PC colleagues.
In November, InternetWeek had a review of five web conferncing systems, and last month Network Computing had a good article "TechU: The World is Our Campus" where they graded nine web conferncing systems.