I've been looking for a free or low-cost solution to web conferencing for some time.  I ran across dimdim a while back and haven't had a chance to implement it (yet), but plan to in the near future.

 

I've been using Logmein.com for remote control to handle customer support, and it works well, but dimdim can be used for both support AND web conferencing/sales/training.

 

What I like about it?  They have a free version that allows up to 20 people in a web conference, using dimdim's central server.  They also have an open source version you can install on your own server so you have complete control, and no limits.  Of course they have a "pro" version that costs money, if you need more features or higher limits, etc.