Hacking Chinese

www.hackingchinese.com

It's full of great advice for how to study the language at all levels, and also hosts monthly challenges that rotate between reading, listening, speaking, and writing. The articles can be searched by level (beginner, intermediate, advanced) or by skill (listening, reading, speaking, writing, vocabulary). I've been referencing this blog for at least 5 years now and I love it so much!