I’ve used the Authorstream web application to post several slideshows on my website. It’s quite easy to use.
Now they’ve added a new feature called Present Live. You can quickly upload a PowerPoint presentation and then show it in real-time over the web. A chatboard is connected to it so you can communicate instantly. You can read more of an explanation on how it works over at Mashable. Authorstream itself also has a nice screencast about it.
It’s not quite “collaborative” since others cannot actually change the slides as they are viewed, unlike some other applications I included The Best Online Tools For Real-Time Collaboration, which let you make the presentation together. But it’s close enough, so I’m adding it to that “The Best…” list.