Quietube is a simple service I built for watching Youtube and other online videos without distractions. It then got rather big.
It uses bookmarklets, auto-generated short URLs, switchable CSS views and user-generated Greasemonkey scripts to give visitors a range of viewing and sharing options on a lightweight framework.
After launch, Quietube was quickly featured on CNET, Lifehacker, Buzzfeed and other popular sites, gaining hundreds of thousands of visits in its first month. It seemed to strike a chord, and is ongoing. Even William Gibson uses it.
As of Summer 2010, Quietube attracts around 1.5 million visitors per month. Many of these are from the Middle East: I've written more about this at Booktwo.