We keep an updated eBook of the most important math formulas just to answer this question! You can download a PDF or read it in its entirety here.
In a pinch or hurrying through your last week of GRE review? We have a formula cheat sheet for that too! Check it out!
We have a dedicated formula page for our GMAT students that is a comprehensive formula source. Everything is broken down by category, so you can find the formula you need quickly! If you're on iOS, we also have a GMAT math flashcard app just for these formulas and other quick math strategies :) If you just need the most important details, check out the top 5 formulas that are must-know for your GMAT test day!
Finally, one area that many students come to us with concerns about is geometry. Because of this (and because it's always good to review in targeted fashion!), we've also put together a geometry formula page which you can access here.
Have a specific problem type in mind, rather than a formula? Don't forget to check out our GRE Math blog as well, which has a comprehensive database of posts, practice questions, explanations, and all the GRE quant content you can imagine!