Magazines will only get you so far, Google searches don't usually yield technical papers, or not many or the best or ones that can be read in full. NASA's public website is also limited and somewhat general, ESA's too. Yet many times when I've asked a question here I am directed to a pdf of a great technical paper on exactly that topic, sometimes within minutes.
Through this assistance I have found a couple of mother lodes. There may be more, and there may be an approach to using them I haven't figured out. What are the key resources, and are there any important tips on how to use them?
Edit: this question has been designated a community wiki. So, go ahead and add whatever you've got to the one answer below, so the resources are all in one well organized answer.