The Slackware ecosystem is not a centralised entity like those you might be used to with other operating systems. Its life is far more akin to the early frontier days of the Internet than modern homogenised corporate franchises.
That can be a little confusing at first, and a little lonely, since most of the really loud noise on the Internet and in computer stores and magazines point to stuff that doesn't apply to you as a Slackware Linux user.
These are well-known, trusted, and reliable Slackware Linux sites, more or less in order of relevance to Slackermedia:
Multimedia and Related
These servers distribute software of interest to Slackermedia users.
http://slackbuilds.org: the go-to site for Slackware build scripts. Contains far more than just multimedia applications, but is easily the “official unofficial” home of installer scripts for Slackware.
http://studioware.org/packages: the go-to site for pre-built Slackware multimedia applications. It doesn't contain all multimedia packages by any means, but the ones that it does are just too easy to install to be ignored. A dedicated team builds and tests these applications year-round.
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http://repository.slacky.eu: assorted builds and scripts, some multimedia applications included, without the restriction of USA restrictions.
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General
These servers distribute generally useful software.
http://www.slackware.com: technically, this is obviously the mothership, but largely inactive except around release time. Mostly, anything you can get here, you can get from your Slackware install media.
http://www.slackware.com/~alien/slackbuilds: a great site for assorted apps and scripts. Due to its location, this server is subject to USA lobbyists and patents, so it is primarily a non-multimedia application source.
http://download.salixos.org: a good site for assorted builds, with a lot of duplication from the main Slackware set, but with some extra gems here and there.
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Advanced
These servers are geared toward advanced Linux users, not beginners or full-time artists on a deadline.
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http://alien.slackbook.org/ktown: for advanced users, this site periodically offers KDE updates. This is not generally advised for most users, as a major update of this type could break things.
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Developers
These servers are geared toward developers, not working artists.