alt Hacker NewsStack Overflow is CC licensed
> Content contributed before 2011-04-08 (UTC) is distributed under the terms of CC BY-SA 2.5. Content contributed from 2011-04-08 up to but not including 2018-05-02 (UTC) is distributed under the terms of CC BY-SA 3.0. Content contributed on or after 2018-05-02 (UTC) is distributed under the terms of CC BY-SA 4.0.
CC BY-SA requires attribution. And is actually quite like GPL, in that it requires derivatives to be under CC BY-SA too!