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.