Drive By Malware Downloads

The Malicia project is a dataset of malware binaries collected by a group of Spanish researchers. Malware recognition is an extremely challenging problem. You have to contend with adversarial opponents, mutable code, and a very poor representation of data, of course these same factors are what it so interesting. This data set is free, but as it is literally a bunch of virus access is by request only. You probably will need to be affiliated with a university to be able to get access.

http://malicia-project.com/dataset.html