I have been using GMATclub tests for Quant. It has topicwise questions and answers explanations. It helps you not only strengthen your weak areas but helps improving your timing as well. One can choose from various time selection modes available and select topics from which you want to create, say, 30 minutes tests. Your results are then displayed on the dashboard depicting the time you took on each kind of question.
It also provides an option to post your doubts in the comment sections, for which you get a reply in a few hours itself.
I recommend GMATclub tests for everyone who wants to ace GMAT.