A few weeks ago I wrote a post about WebEx alternatives. My conclusion was that there are some good options for sharing a PowerPoint presentation but WebEx was one of the few services that allowed a non-Windows user to share his screen for a software demonstration. WebEx isn't cheap.
I just learned about a service called Yuuguu that seems to work pretty well. Yuuguu is a free service works like an IM client on Windows, Mac, and Linux. All meetings participants need to download and install the client and register for the service. This is not ideal for one off meetings with people you don't usually work with (like a sales demo). Yuuguu is much more appropriate for distributed teams that work regularly together.
The service pays for itself (or at least intends to - I don't know if they are profitable) by recommending a free conference service (British Telecom's Powwow Now) to use during your meeting. If you don't pay extra for your long distance dialing into the free conference number will not cost you anything.