I'm not sure about everyone, but I personally have a lot of issues with OAuth based authentication in API's, especially when there are no high level libraries (which is true for Tumblr). And I know from first hand experience that dealing with Tumblr's API, where they tell you auth is OAuth based and that's about all, it gets a little confusing when you don't understand exactly how everything works. My experience with authing with Facebook and LinkedIn have been pretty good since they will walk you through how to do it and even add some abstraction for OAuth in their libraries.
But for all the slightly-confused devs out there who aren't that familiar with OAuth, Google has launched an OAuth Playground. Here you can play around with calling for authorization tokens, access tokens, request tokens, whatever other tokens, and all for whatever API you want. They have some pre-loaded ones, but you can copy paste your own in. This will let you learn through experience how the whole OAuth process works (with all the back and forth calling/requesting) and let you do some test runs on whichever API you plan on using for your next project!
I am definitely going to use this for whenever I have my next project that requires OAuth authentication. Since I am much more of a learn-by-doing than learn-by-reading, I think this will help educate me for all future projects too.