I'll be honest...I never really 'got' the PDP-8. It's a great example of what really, really smart folks can come up with when you have a very limited transistor budget. The ISA is limited and quirky (aka 'strange'), there's not much memory and it's relatively slow, but a huge amount of serious work got done.

OS/8 and TSS/8 are the 'common' OS. ASM, FOCAL, FORTRAN & BASIC were popular programming languages. You can find images lots of places. I don't guess there's a lot of 'canned' software, as the PDP-8s were more targeted at roll-you-own software. I personally think the most interesting things about them was the many places they were used as controllers for other things, like manufacturing machines and laboratory kit. Not very amenable to a home lab, but they have a very dedicated following in the classic computer world.