slackbuilds.org - This site holds community submitted builds scripts to easily build Slackware packages from the source code of various applications.
slackware.it - This is a repository of pre-built packages compiled in various languages.
alienBOB's repo - This site contains pre-built packages and the associated build scripts used to build them, so you can use it to download packages or to get stable build scripts and build the packages to fit your needs.
If you would like another repo added to the list please propose it inthe
slackdoc mailing-list.