Mac OS X 10.5 Leopard promises to provide screen sharing, but for the next few months, or when needing to share screens with people on other platforms, take a look at, which provides  in any Web browser. I ran across it a while ago, and have used it once successfully when getting a demo of SpotDJ, though on another attempt, Jeff Carlson wasn't able to get it to load. Basic features available for free (with ads) for up to 10 users include desktop sharing, free teleconferencing, annotation and whiteboarding tools, the capability to change who's presenting, and public and private chatting. You can pay to increase the number of simultaneous users, and also to enable features like the capability to share mouse and keyboard control with other attendees; scheduled sessions; 100 MB of shared file space; and Web session recording, playback, and hosted webcast (they're all available for 15 days for a free account). Honestly, I'm hoping that Leopard's screen sharing meets my needs, but if it doesn't, I'll give Yugma another try.
Oh, and if you were wondering about the thoroughly odd-sounding name, Yugma means "the state of being in unified collaboration" in Sanskrit, one of the classical languages of India.