Last Dreamforce (Oct. 2014), Salesforce announced a new interactive way of learning all about the Force.com platform through a new resource called Trailhead. I have reviewed a lot of online training platforms and I have to admit that Trailhead is the best free online resource I have ever seen. And just last week, they released a new training module on how to build modern apps using reusable Lightning components.
What makes Trailhead, and especially this Lightning module so special is that it challenges you to complete exercises within your own development org. And most of all, the challenges are just that. Challenges. They actually take effort and thought. They are NOT just a step-by-step tutorial re-hash like so many online tools are (not that there is anything at all wrong with that style of learning).
They provide a list of specific end goals, but they do not tell you exactly what to do to accomplish them. You have to figure that out for yourself. Typically, you will have to re-read through the module material you just read, but it is possible you may also have to do a little side research to come up with a solution.
Each challenge takes about 30 minutes, but it may take a little longer if you are brand new to the material. And, each challenge gets progressively harder to complete, so they kind of ease you into it.
If you looked into Lightning and can see what a game changer it will be to mobile development, you really owe it to yourself to go through this training module and earn your Lightning badge. I am half way there myself. Let me know if you get yours and what you thought of Trailhead. Personally, I think it is amazing that Developerforce offers this kind of high-end stuff for free!