I guess everyone had some website / blog hit by Panda / Penguin, however few ones have courage to say it.
Well, I have courage to say, because Google stole me some good money!
Ok, not crying now... If it happened to you, keep calm and do like us:
If you had dozens of websites, maybe you need abandon some of them - now, you need focus more on few websites, so you'll can offer more features and quality content to your visitors;
Review your marketing strategies, MAINLY about linkbuilding. If it looks like spammy, so it's spammy for Google;
Try to get good quality backlinks. And it's more difficult part, because great backlinks aren't easy!