My site that was penalized is The Online Slang Dictionary. http://onlineslangdictionary.com/ It's the oldest slang dictionary on the web and had the only freely accessible slang thesaurus on the web until about a month ago. In contrast to other slang dictionaries, The Online Slang Dictionary is only for legitimate slang words.

I've never used any black-hat or even slightly questionable SEO techniques or means to publicize the site. It started being penalized April 11, which is the launch date of what (IIRC) is being called "Panda 2.0". I've made many changes in hopes of fixing whatever it is that's causing Google to decide that the site is low quality, but haven't seen any improvement in ranking since then. A (now outdated) list of those changes is here: http://news.ycombinator.com/item?id=2664346 .