This is a tool, which shows google / organic traffic from Google Analytics combined with major algorithmic updates of Google.com:
Panguin Tool | Technology by Barracuda Digital
Does it happen to show large traffic drops on the exact dates (lines indicated there) for some of your sites that may have been affected, or not necessarily?
I noticed large traffic drop in one of my sites, but it does not coincide with algorithm update lines (it was a month or so earlier than Penguin indicated there, lets say).
Asking just out of curiosity, maybe you can check.