|Pathologically Eclectic Rubbish Lister|
I've also read Gentoo is similar in that everything is built from source
Well, Slackware is essentially a binary distribution (sources included), but everything not in Slackware generally has to be compiled, or you have to find a binary that runs on Slackware (this is how I use VirtualBox: Oracle offers an AMD64 binary that just runs on Slackware).
Gentoo has a tiny bootstrap system that is a binary distribution, and then compiles everything from there on, with aggressive compiler optimization for the target machine. Gentoo has a lot of "ebuilds" that are essentially build instructions like slackbuild scripts, but already included with the distribution. Technically, you should be able to build Gentoo even without that bootstrap system on any other Linux.
FreeBSD has a similar concept, somewhere in between Slackware and Gentoo. The basic system comes as binaries + source, and it has "ports" built into the distruibution, again a set of build instructions for 3rd party software. The main difference - except for the obious Linux vs. BSD - is that FreeBSD does not try to micro-optimize for the target machine. Other BSDs have copied the ports idea.
T2 and Linux From Scratch are examples for distributions that are essentially only build instructions for a distribution. You can customize virtually everything, and build your own Linux to match your very special needs - e.g. for embedded systems or high performance computing.
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)