GMATClub is a great source for questions that are at par with the actual test. The discussions on the forum are very useful. When you go through the entire discussion, you get to know the other perspectives and methodologies for each question type. Not just that, search any question or concept and you will find it on this forum. GMATClub Quant CATs are also very helpful. It's one of the best ways to practice. These tests provide good analysis of the performance. The flexibility to build your own quiz (topic-wise) allows to practice only the weak topics. Overall a very good forum.