I personally use the MIT license for most of my open software, which is pretty normal for a lot of code in the game dev industry. As far as jams go, it is frequently up to the creators or the jam creators. I have hosted game jams in the past and I am usually specific about requirements: 'Use license X if you want to participate in this jam' or 'we don't care if you release the source or not'.
Creative Commons is great for art (which I don't really distribute as much).