GameAnim.com is a website created and run by Jonathon Cooper (Naughty Dog). It has a massive selection of animation resources from interviews, tips & tricks, showreels and much much more!
Syncsketch is a really useful free website for posting work and receiving feedback. Just upload a video and allow others to review, draw and make notes.
Artstation – Mostly dominated by artists but still a good place for animators to upload their work
Vimeo – A great website for submitting animations, creating a profile and connecting with other animators