I assume you're looking for a tool that can share what's shown on the presenter's screen with other users in real time. There are a number of good web-based tools for this, like WebEx, GoToMeeting or JoinMe. They have free and "pro" versions, the latter of which are fairly robust (I personally happen to prefer JoinMe).

There are also much more sophisticated, video-conferencing based solutions with all manner of presentation sharing capability. Polycom (the maker of the "star" conference phones) has one of the more sexy solutions, which they purchased from HP a couple of years ago. Hope that helps!