I'm going to share some websites that have been especially helpful. Feel free to add your own!
This is a wonderfully witty and helpful site by Catherine Ryan Howard, a self published author. She's awesome! Check her out!
Ok, so this is kinda of given, but the writer's digest website is a good place to practice writing prompts, get feedback, and learn what agents are looking for in new talent.
For the ebook publisher:
This is a site by Susan May Warren designed to help emerging and advanced writers get even better at their craft. There's information about contests, conferences and you can get active in their online community.