Not a comprehensive collection; there are repositories of classic winamp skins scattered throughout the web. I myself still keep copies of every skin I've used in Qmmp.
deviantart.com use to host plenty of winamp classic and modern skins (version 5+) but sadly, it's no longer publically browsable. Others like wincustomize.com and customize.org already have snapshots on the Internet Archive's Wayback Machine.
