Tvtropes, a website located at http://tvtropes.org, is a user-authored website based on a wiki engine, exploring the usage of tropes in fiction. The site was originally focused around the use of tropes in television (thus the name), but it now encompasses almost all forms of fiction. Being on the intarweb, and thus home to many geeks, there is much focus placed on anime, manga, and video games. However, there is also some fairly intelligent comments made about any number of mediums, including classical literature.
The way the site works, is that each entry focuses on a trope, a work or an author/creator. A trope, for example, "mad scientist", will describe works in which it is present. An entry around a work, such as Star Wars, would list the tropes present in that work. An author page would describe tropes that the author uses. The tropes run the gamut from fairly obvious, widespread things (such as "Mad Scientist" above), to things that are more obscure and need more explaining (the "Cue Cullen" trope). The site explains on its entry page that "tropes are not cliches", and that listing a work as being full of tropes is not to say it is cliche or derivative. The site is actually quite insightful and complex when examining the art of storytelling.
tvtropes is also a very fun site, both to read and to contribute to. The discussions can be very freewheeling, and there is a lot to add. It is also a prime example of a site that brings back the halcyon younger days of the internet, when randomly clicking on links with glassy-eyed fervor could wrap the user up for hours. tvtropes is definitely a site that leads to wild tabbing (as mentioned in an xkcd comic). Outside of my obviously favorite user-contributed website, tvtropes is one of the funnest, most informative sites available.