Collabedit

Collabedit (http://collabedit.com/) is a free, real-time, browser-based collaborative source code editor. What does that mean? It means that two or more users can work on the same source code document in real time, on-line, and any changes made to the document will appear in real time as they type. Collabedit will automatically color code the syntax of your document. Codes supported include C++, basic, visual basic, css, html, javascript, plain text, and many others. There is no limit to the number of users, and no login is required.

To begin, you simply click the “Create a New document” button and Collabedit assigns a URL to you. You then send the URL to whoever you wish to collaborate with, and begin building your code. When you are finished simply click the "download" button to download your work.

Did I mention it's free?

Here is a screenshot of a css document being coded by two users: