A great example of this is Wikipedia. Other than the occasional fundraising banner at the top of the page, Wikipedia displays content front and center, leaving the user interface out of the way. This is not to say that Wikipedia's navigation UI is neglected; when needed, the search bar is easy to use and navigation links are perfectly accessible at the side of the page. Wikipedia's UI detracts from the content in no significant way, making it an excellent site for sharing knowledge.

Google also serves as an example of simple design done right. The search bar, being the gate to content, is one of the most prominent features of the otherwise bare home page. Start searching for something, however, and the information you seek quickly moves to the forefront, shunting the less useful search bar to the top of the page. This use of simple design is what makes Google the most popular search engine in the world. Whereas Yahoo's purpose is hard to understand (is it a search engine? a news site? a web portal?), Google is very obviously a search engine. This is, at least to me, the ultimate purpose of good design: to make the usage of the product obvious.