Even though I've been doing Android development for over four years I still come across neat things I never knew about Android every month. For example, this month I discovered Activity.startActivities() - how it took me years to find out about it I don't know. It is embarassing and once I learn of their existence I start using them, cursing myself for failing to discover them earlier.
So I'm going to start a new project called the Android tip of the day. Each day I will post a link to an Android class, method or attribute on Twitter and Google+. I think this project will be great for self-learners; I'm just going to point you towards something you might never have seen before and could use in the future. Things that don't require a full blog post to explain - you just needed to see them. Hopefully I'll find a few cool new API methods myself in the process.
I'm going to keep this project going until I run out of cool things to point out on Android. That will take longer if you help me out with suggestions, so please let me know of cool Android APIs!
Edit (May 28, 2014): The project is now complete - check out the five round-ups for a summary of the tips posted.