TheNLP resources list on github is a greate place to start. (We apologize for any repeats/overlaps with our list here)
Interactive Stemming and Lemmatization Website here.
Our recommended beginner's NLP bible, the NLTK Book, by Steven Bird, Ewan Klein, and Edward Loper.
If you're looking for a real challenge, Stanford provides a Natural Language Processing with Deep Learning course, which appears to be fully publicly accesible. The syllabus.

Have a great resource you think should be on this list? Shoot us an email on the contact page with the link or resource info!