The even shorter version: We do other compos too now!
The short version: In a sample pack contest (also sometimes called a compo after the demoscene term), the entrants are given a set of samples and are required to make music using only those samples. The community then optionally votes on a winner. When the dust settles, the tracks are all compiled into an album, which is released on Bandcamp. Absolutely anyone can join, and sometimes they do.
The rules may differ slightly from compo to compo. For the rules of the most recent contest, check here.
The first contests were hosted by Dave Harris on VGMusic.com's forums, but those no longer exist, and instead we're on its Discord server.
This website is an alternative to the Bandcamp pages, with the goal of eventually having a more complete archive of SPC-related materials. It is hosted by Shane Evans. In case anyone wants to help support hosting costs (hey, I can dream, right?), I've provided Paypal and bitcoin links below.
If you have questions or comments, the best way to reach us is at the aforementioned Discord server. If you don't want to use Discord, you can also send an email to spc at beam saber dot com.