This is mainly for personal reference, but I am sure you guys will find it useful, too. :)
I will add to this whenever I find some new ones.

General tips and good practices

• PSG Art tutorial
  A good reference of important concepts in art. Now mind you, it just quickly introduces topics on which whole books have been written, but I personally found it invaluable. Good reminder on things I tend to forget.

Faces and Facial Expressions.

• 360s facial expression references (female)
  The free not-book-to-buy version has only eight basic expressions, bu it’s still really useful to figure e.g. where the nose goes when people scream their head off.
• for the love of noses
  photo references of impressive human… noses

Bodys and Bodytypes, Anatomy in general

• Pixellovy Figure and Gesture Drawing Tool
  Photoreferences, best thing since sliced bread. Intended for practice, free to use.
• athletic body diversity reference
  (non-nude) photographs of a whole bunch of olympia athletes from a wast different array of sports. Nice to check the differences between a swimmer and a runner, or between a gymnast and a ringer.
• The Drawing Script
  This is more intended for practice than for reference; it gives you random photos of your choosen subject matter and switches after time is up. The photos are pretty high quality and you can set greyscale and things. Plus. They have big cats. Gotta love big cats.

Staging, Layout, Poses and other Comic specific tips and tricks

• Comic Strip Artist’s Kit (Redux)
  Some basic knowledge about comic strips and panel layouts, pretty useful, actually. I’ve got a mirror on this site, but you really should check out the source.
• Less Tits n’ Ass, More Kickin’ Ass
  Redraws questionable US-American Comic book poses into anatomically sound artwork with a focus on the female characters.
• Posemaniacs.com
  Features renders of models (both male and female) in lots of poses, plus a nice “30 seconds for a pose” feature that’s clearly meant for practicing art.
  Biggest drawback? The majority of female poses are shitty “sexy” ones, so be aware of that.
  And well… It’s a good pose reference, but due to the limitations of 3d models, it’s absolutly fracking useless for anatomy. So, do NOT use it for anatomy references!

If any of these links isn’t active anymore, please shoot me a short email, so that I can take it down.