It seems Google has rolled out another panda update yesterday....
One of my site (highest earner) was hit yesterday. It was on first page of google for around 200 Long tail keywords related to each other. But suddenly it disappeared from top 100 results for almost all 200 Keywords, in fact it is on 5-6th page for 2-3 Keywords out of those.
This site has survived all the panda updates till yesterday and I had increasing Traffic day by day till yesterday, moreover I have not done any link building for this site in last 1 year or so... Whatever hit it yesterday, it is really not good, now I see lot of spammy sites on top 2 pages for almost all of these keywords.