Hot Topics in English Language Learning

Today, instead of answering questions, I'd like to ask one:

If you could go online (using a computer or mobile device) and download mini-programs/applications (apps) that just focused on one specific topic, what would it be? E.g.: maybe you want help learning English articles, learning TOEFL vocabulary, or improving your pronunciation of the sounds "L" and "R".

What are the first topics you would search for, and why?

I hope this will be the start of a very interesting discussion!