Skype's new group video conferencing is super buggy, but I have gotten it to work. Normally it takes 10-15 minutes of screwing around and making sure everyone has the latest version. The UI sucks on the new version, but after a while you can get used to it.
For one-on-one sharing Skype is fine.
A free conference call system + Google Doc with an agenda is also a nice option.
finally, Adobe Connect seems to be getting a popular. I've had an OK experience with it one-on-one. I think it's free.
Basically if you're doing this professionally and more than once a month I think GoToMeeting is worth it.
Slideshare just launched PowerPoint sharing with video with their Zipcast features.
Salescrunch is about to have some really cool tools for more sales oriented presentations