There are three main libraries for property-based #testing in #Erlang:

• #QuickCheck — the original implementation, proprietary;
• #PropEr — free, but viral (#GPL);
• #Triq — free, Apache License 2.0. (I’ve been recently granted commit access to the leading fork).

I guess that the fact that PropEr is significantly more popular than Triq is partly thanks to people not paying attention to #licenses at all. That’s unfortunate, as more man-days are put into a library with a problematic #license.