Free Tools for Collaborative Editing?

zachrahan asks: "I have almost completely removed Microsoft Office from my work-flow. One hurdle remains, though -- sending scientific manuscripts out to colleagues for comments. Everyone I know simply uses MS Word's Track Changes feature for this. To tell the truth, this works quite well. However, I'd prefer to use free software to write my articles, like LaTeX or OpenOffice and then distribute PDFs or host HTML files for people to look over. I've been working a bit with Multivalent, which is very promising, but still firmly in alpha. Are there any other free, cross-platform tools for collaborative marking up of PDF or HTML (or other) documents, a la Word's track changes feature?"