A collection of botnet source codes, unorganized, for educational purposes
When looking for malware and especially virus source codes for research purposes, you shouldn't miss the (sadly offline) VXHeaven library [0] and any ezines of VX groups [1].[0]: http://83.133.184.251/virensimulation.org/src.html
[1]: http://spth.virii.lu/ezines.htm