Web Conferencing
So far I've found Webex appears to be the big player (before you go into really heavy stiff with dedicated hardware). GotoMeeting is similar but doesn't have video. However, I also stumbled across WebConference. They're much cheaper than Webex, and a brief try-out suggests they could be what I'm after.
Webex is platform independent, and Webconference is almost so - you can't share a Linux or Mac desktop, but you can do the video/audio conferencing thing.