There are tremendous causalities with this latest Google algorithm update targeting "webspam" named Penguin - tremendous.
Almost 65% of those who responded to my poll, about a thousand of you, said you had a negative impact with this latest update. Trust me, I feel really bad for you and if you are squeaky clean, do submit your site to Google for review.
If you were not hit, do you want to see what it looks like? There are many threads with charts and such of traffic drops, I picked out two at Google Webmaster Help.
(1) What the traffic drop looks like:
(2) What the keyword ranking drop looks like:
Scary looking charts, don't you think?
Forum discussion at Google Webmaster Help.