The problem: Our trainer had a MacBook and no connector to use our projector, and she couldn't use our Windows-based conferencing software. 

The solution: Yuuguu. It works on Windows, MacOS, and Linux, and with IE, Firefox, and Safari. Only the host needs to install anything; participants simply browse to the meeting URL and enter the correct pin to join in -- a fabulous feature, as I didn't have admin rights on the computer in our meeting room. If I were able to install Yuuguu, we could have gotten control of the web conference and shared our desktop, for full collaborative work. 

Yuuguu does offer fee-based conference calling to supplement the presentation, but we used a separate conference call and could have used the integrated instant messaging (useful for a presentation for which participants are muted on the call). Later they'll roll out professional/corporate level features and pricing, but they promise to keep this current functionality forever free.

It's a great tool I'll be turning to in the future!