If you're in need of a quick and simple way to share a document with a few other people at the same time, worth checking out is ShowDocument. It works with most major file types including PDFs, Word docs, PowerPoint presentations, and text files, and displays them in a simple Flash-based viewer and editor.
Once you've uploaded any file up to 2MB in size, you can mark it up with a pen and highlighter tool, along with a text box tool and eraser. There's also a small chat box to discuss changes, and everyone's cursor is tracked as it moves around.
Everything is delightfully simple to use, although one big problem we ran into with our testing was that anyone can control what's going on. This cropped up the most when one person was making an edit and someone else decided to take the reins and scroll to another slide. Whatever edit that person was making got ditched and there's little else you can do besides yell at them through the chat tool.
Ideally, future versions would allow some sort of presenter/viewer dynamic to keep someone else from overtaking the editing experience. To the service's credit, very few live collaboration tools handle user edit switch-offs with grace.