Skim Knockoff(s) on the Mac App Store
The app listed for sale for $29 on the store as “PDF Reader” is actually our free and open source Skim app, although no mention of the Skim Sourceforge project is made in the description, and the support link just goes to a livejournal post where the Skim web page ad copy has been pasted. There is also a $19 app called “PDF Expert” that appears to be another copy. That one even went ahead and used Adobe’s Acrobat trademark in their icon!
Skim’s BSD license allows others to use the code, but not without attribution. The Skim developers have not had any luck getting Apple to do something about this.