I've had numerous ideas for web-based applications and the folks behind Writely have come pretty close to reading my mind.
Writely is an online document editor. It's primary purpose is to facilitate document sharing between multiple authors. That in itself is interesting. With Word support, it can replace the need to purchase Word by allowing document editing via the browser.
Another feature they should look to implement if they haven't already, is PDF support. Being able to generate PDF's on the fly and e-mail them off would be fantastic.
From a marketing perspective, I likely would have focused on the site's ability to replace the cost of software like Word and Acrobat as opposed to the document sharing aspect. I'd be interested to hear if they contemplated one approach over another.
It's still in beta and have been seeing a surge in traffic. As a result, they are currently not accepting new beta testers. I look forward to getting the opportunity to try out what should be a rather intriguing application.