Here's a good one. Webex is a pretty decent web conferencing solution, and I've used it plenty of times. However, it isn't free, or even particularly cheap. If, like me, you only need to work through presentations and files in real time once every few weeks, there's no point shelling out.
DimDim is nice and functional open source alternative, and pretty simple to use. Unfortunately, it hasn't got full presentation support for Linux at the moment, but when they sort that out, it'll be even more appealing.
