Another site relevant only because it pioneered concepts is the to-do-list management site called Tadalist:

http://www.tadalist.com

The site claims 4 million to-do entries, which is quite modest compared to the number of users of popular sites. But the relevance here is that it was one of the first web sites to push the idea of a simple and focused web-based application replacing a corresponding dedicated or custom desktop program, and also the use advertising to support the site, allowing users free access.