I can point to two resources which just became available in 2014, which I've found useful:
BenchPrep's FREE GRE Playbook (100+ pages in PDF format): https://benchprep.com/gre-ebook
The 20 Most-Difficult GRE Practice Questions (also in PDF; they're pretty hard but you can see how the test-makers think): https://benchprep.com/hardest-gre-questions
Magoosh also has a lot of free eBooks.