Sure!
I'm going to make a distinction between study and practice. Study is when you learn the underlying techniques, content, and strategies. You're mostly learning this from test prep company materials. Practice is when you take what you learn and try to apply it to test questions under test-like conditions; the best source for this is GMAC.
For study materials...I am obviously biased and like my company's materials best.
But I will tell you the kinds of things you may need and you can examine stuff from different companies to see what you like best.
You need something to teach you techniques for the different question types (eg, techniques for working through SC, CR, RC, DS efficiently and effectively). I left PS off that list because it's the most "straightforward" (initially) of the types. Then, you also need strategies for handling particular kinds of questions - eg, smart numbers, working backwards, and testing cases for quant (the first two of which are exclusive to problem solving).
You also need to learn the underlying quant and SC content (facts / rules) that the GMAT tests.
As a baseline, you're going to need access to some books / lessons (whether physical or in e-form) that will help you to learn those things. You might do a self-study program or a class...that's up to you.
I don't know how far you are in your studies, so it may be that you need study materials only for certain areas.
For practice, you'll want to work quite a bit from GMAC materials, as their problems are real GMAT problems (though I'll warn you that their explanations are not always the greatest). I recommend that everyone use, at a minimum, the latest big Official Guide book (~900 problems) and the free GMATPrep software (which includes 2 full tests and about 90 free questions).
Beyond that, there are two additional OG books, paid add-ons to GMATPrep (a 400-question set, 2 more practice tests), GMAT Focus (quant), and GMAT Write (essay).
Of those, I think the absolute best product is GMAT Focus, because the questions are phenomenal and it's the only way to test your quant adaptively without having to take a full practice test. It's quant-only, though, so not necessary for someone who isn't struggling with quant. (I wish they had the equivalent for verbal!)
If you use GMAC materials (which I definitely recommend!), then you'll also want to identify places to find better explanations. As you've seen here, we have folders for the free GMATPrep questions. A number of the GMATPrep questions have also been dissected in blog posts, so you might want to search our blog for the relevant text. We also have a program called GMAT Navigator that includes written or video explanations for a good percentage of the problems in the big OG (and we're slowly and steadily adding more over time). This program has a free version (that does not provide access to all content / features) and a paid version (which does provide access to everything), so you can try the free version to see whether you think you might want access to the full version.
What do you think about all of the above?