If you like Google Wave's real-time collaboration but not the interface or registration, check out TypeWith.me. It's a solid, simple, as-you-type document collaboration web app with no sign-up required.
A quick background: Google recently bought previously mentioned EtherPad, then closed the service. Then, after some backlash, open-sourced the entire app. TypeWith.me took that code and put it on a new server that won't be closing down anytime soon.
Getting started with this free app is as simple as clicking the New Document button on the site's home page. You're instantly dropped into an edit-ready page where you can start typing right away, or import an HTML, plain text, RTF or Word document. There are a few standard text formatting buttons to choose from, but this app is really all about the text.
Each online document has its own unique URL that you can IM or email to others for co-editing, and there's a small window for chatting on your edits. TypeWith.me helpfully assigns different colours to each author so you can easily track who's typed what, and the app instantly synchronises as you type so changes appear in real time. When you're done, export your work to one of six file formats, including Word, PDF and HTML.
Google Wave users will see a lot of similarities between it and TypeWith.me, but this simple app has an easier learning curve.