Practice questions

The Official Guide to the GRE General Test - 3rd Edition (paid)

This resource is an absolute must for anyone preparing for the GRE. It features hundreds of retired (once on the actual test) practice questions. So, they will give you the best idea of the types of questions you’ll encounter on test day. You also get four full-length practice tests—two in the book, and two on an enclosed CD-ROM. 

Official GRE Quantitative Reasoning Practice Questions (paid)

This book features 150 retired GRE Quantitative questions and solutions. 

Official GRE Verbal Reasoning Practice Questions (paid)

This book features 150 retired GRE Verbal questions and solutions. 

GRE Super Power Pack (paid)

Save money by purchasing the 3 above resources all in one book

GRE Prep Club Discussion Forums (FREE)

This popular GRE discussion forum has over 16,000 GRE practice questions, including tons of official questions. Each question is tagged by level of difficulty (a feature I particularly like). There are tons of free resources, and the site has a timer for each question so you can see how you're doing time-wise.