TriSamples is one of my favorite resources when it comes to quality, free drum kits and sample packs. Though they don't have a ton of kits to choose from, to my experience they are all high quality. The kits that I've personally downloaded and used are the 808 Trapstep Pack (volumes 1 & 2) and the 808s in the Box kits. All kits are Windows/Mac compatible. If you have not heard of the TriSamples, please make sure you go check the site! You never know what you may find.