Slackbuilds.org,
Slacky.eu and
LinuxPackages are probably the best known, there are others.
I personally prefer Slackbuilds.org as you get the slackbuild and source and compile it yourself. Their selection is limited at the moment compared to the other two. Slacky.eu usually provides a slackbuild, LinuxPackages only sometimes. Some packages from Slacky.eu and Linuxpackages can have unnecessary dependencies.