Any code you have. Open source it for all to enjoy.
GPL version 3.0 All you will ever need to know about the GPL
If you would like to know anything about the GPL look there, also read this as it shows you why open sourcing your code is a advantage
http://www.catb.org/~esr/writings/ca...hedral-bazaar/ it convinced Netscape to opensource Netscape back in the late 90's.