What's the Best Development Resource You've Ever Found?
Most days we just read individual tutorials to solve specific problems, or we head over to Stack Overflow if you have a really specific question.
But then there are some resources we find that we bookmark and we reference again and again when learning a new topic, or developing for an entire ecosystem. Some of these resources are really remarkable in the amount of great information they provide and other resources that they aggregate. DZone Refcardz are the first example that comes to mind for me.
What are some of the best developer resources you've ever found?
Share as many resources as you like in the comments! Don't include software documentation. That's a question we can ask at another time.