What websites do you find productive and when, or unproductive and when?

My view is that every application is a tool. It can be used well or misused. Some though have a higher propensity for the latter than the first. What are the webapps you use, when does productivity or usefulness break down – what is your best advice to overcome that?

Use this format if it’s helpful to achieve a more complete answer.
Service name(include link if you want.): Ex Facebook.
1) Positive use(s): Ex reconnecting with lost friends, organizing fund-raising events, finding a mate etc.
2) Negative use(s): Spending too much time looking at pictures, like-ing things, stalking profiles.
3) Ways to overcome productivity loss: Limit time on the website overall, limit time viewing pictures – ask the question – how does using this benefit my todo list and overall purpose(s)?

Other example services. We can use this as a reference – and creative store for productive uses of web-services with concrete examples.