Writely

Writely is a Web word processor. It allows you to share your documents instantly and collaborate real-time, pick exactly who can access your documents, edit your documents from anywhere (nothing to download - your browser is all you need, and store your documents securely online (offsite storage plus data backup every 10 seconds). Writely is easy to use. It implements clean, uncluttered screens with a familiar, desktop feel.

Requirements:

Microsoft Internet Explorer, Mozilla Firefox