I have 2x PCIe X16 and 1x PCIe 1x slots that are not being used.
Given that Linux has extensive hardware support, there are probably users with interesting PCI card usages
Aside from traditional usages like network/wireless/bluetooth/sound that can easily be used as USB (or built in advanced sound support in the MOBO), what are your use cases with PCIe?
I have an extra PCIe USB card I can use for Virtual machine pass through (sometimes it's useful to be able to pass the entire controller instead of just a device).
I have a Hauppage TV tuner card that I use to get OTA TV and record certain shows. I'm pretty sure there are USB solutions, but if I remember, at the time I got this it was one of the only cards that my system supported or whatever caveat I was operating around.
This is a really interesting question. I remember a lot of very niche PCI (not PCIe) cards from the old days.
My current-day boring use-case: Multiple GPUs that have never once been used for gaming. One is an A2000 card that does video transcoding and tone mapping for Jellyfin. The other is an old Quadro card that handles encoding/decoding for my NVR.
You can use a SAS controller or 10G ethernet card in the 16X slots. If your motherboard support PCIe bifurcation, you can get an adapter card to use 4 NVMe drives in a 16X slot. You can get controller cards for older interfaces like firewire, serial, and parallel that will use a 1X slot.
I got a card for an extra m.2 drive, and another one with four usb3.1 ports on a separate controller, although i dont use it anymore as much. Haven't had anything else installed in a while. The last one before that was an audio card I've bought around 2010, and it was a huge waste of money
Additionally, I had a 3 port FireWire card that I used for digitizing old MiniDV camcorder cassettes. But I had to remove it because someone fucked something up in kernel 6.5, crashing the whole boot process after a few milliseconds.
I have a 6700xt for Linux and an old 2070 that I pass through to a windows guest. I might get a usb card too to pass through but not sure if I have any more space in my mid tower for another pci card.
A molex to PCIe adapter with an 850w power supply. I only have like 10 watts of headroom but usually only one gpu is running at a time so it works fine. I would have bought a better one but my dad accidentally bought 2 for another project so he had an extra one and it works fine.