I tend to sign up for anything that looks even mildly interesting, including the Useless Account.  I have found some decent online tools as a result.  All of them have free accounts.  Here are some for creating documents online that I’ve tried:

• Zoho - Similar to Google Docs but some additional features.  I like Zoho’s interface a bit more than Google Docs.  Check out Lifehacker’s comparison.
• Buzzword - A word processing tool from Adobe.  Buzzword is a fully-featured word processor that aims to give the user a desktop-like experience.  Really fancy interface.  Here’s a review.
• Writer - Writer is an online version of the distraction-free Writeroom.  Nothing fancy…just type.
• Gliffy - Great for creating Visio-like diagrams.  You can export to SVG which Visio can read, too.
• Writewith - Aimed at collaborative writing.  Chat and version history of documents are really neat.