On: Going where BeOS NetPositive hasn't gone before: NetPositive+

[nickt]

https://www.vogons.org/ For the more mainstream retro machines and hardware.

https://forum.vcfed.org/ For the rarer esoteric stuff (but still loads of old Macs, DOS machines, Amigas and 8-bit stuff)

There loads of others, often machine specific, but these 2 are a good place to start.