Depends where you draw the retro line.
I've a Dell socket 370 in the garage I picked up for free and done nothing with. That's as old as I can go
I've a few spare ATX cases, PSU's and Socket 775 M/B's maybe a 478 as well if I have to build from scratch.
Loads of CPU's
Plenty of AGP,PCI and 1 or 2 PCI Express and ISA graphics cards.
Only have PCI sound cards.
Most of this hardware isn't anything special though mostly leftovers from upgrading my existing retro machines. Only thing I can see been used in a permanent build is the Dell
Also quite a few complete socket 1366 workstations which I do intend to do some builds but prices haven't come down enough to start those projects yet.