SCBWI: The Society of Children's Book Writers and Illustrators - Requires a membership fee, but if you write for children, this is the most useful organization you can join. I participate in my local (Nevada) chapter. They offer in-person and online events, critique groups, conferences, and lots of help through the writing and publishing process.
IWSG: Insecure Writers Support Group - A place for writers at any level (beginners to published and beyond) to receive guidance and overcome insecurities. They also have an annual anthology contest. Pitch Wars - A mentoring program contest to help writers improve their manuscripts and get published. They also host quarterly Twitter pitch events called #PitMad.
Twitter - Follow authors that write in your genre, participate in pitch contests, or look up agents to view their current #MSWL (manuscript wish list).
Publishers Marketplace - A marketplace for publishing professionals. Members have access to additional features. A great place to find listed agents--agency research is free.
Your Local Library - As a librarian I would be remiss if I didn't mention the great options libraries offer writers. In addition to free books on writing and publishing like The Writers Market, many libraries offer local writing workshops, contests, and community support. They're also a great place to sit and get writing done.