Here are my all time TeX.SE favorites. TeX.SE became a very reliable and valuable source for TeX related questions. Beside the new approach to a internet driven Q&A site which seems to be very efficient — maybe due to the fact that moderator intervention is not needed so often — a lot of TeXperts (as Knuth calls TeX Programmers aka package authors) are around there. And this makes some answers really precise. Now see my list:
Basic understanding
Just nice
More on graphics in pdfs (with pgf)
Meta topics
… to be continued