I know that Gmail and google+ were all written using google closure, but are there any good examples of websites outside of google that have been made with google closure?
Zoosk uses Google Closure. I think that one reason why Closure is mostly used in-house is because it leans more on the side of handling the complexity issues of large scale web 2.0 application development over ease of use or quick learning curve. As more tools become available for building Google Closure apps, I think that you will see more adoption outside of Google. For example, there is an application generator that creates Google Closure apps from UML models.