For people that aren't getting free boxes from work, you can usually find used micro/tiny form factor Intel machines for around $120. That's more than a Raspberry Pi, but you can get a Core i5-6500T with 8GB of RAM, 256GB SSD, and it'll use around 10 watts at idle (and top out around 50 watts). You'll get an M.2 PCIe 3.0 4x slot and a bay for a 2.5" SATA drive and support for up to 32GB of memory.

You can put Proxmox on them if you want to run a bunch of VMs or use Windows or any of the many Linux distros that support x64 rather than dealing with less known hardware.

While 10 watts is more than the 1-2 watts of a Raspberry Pi, it comes out to around $13/year for me leaving it on 24/7. A normal desktop idling at 50-100 watts would cost me $65-130/year so it is a big difference. In fact, if you're leaving a normal desktop on 24/7 as a home server, it might make financial sense to grab a micro/tiny machine. If it costs you $120 and saves you $50-120/year in electricity, that seems like it would be worthwhile.

They're not fanless, but they are very quiet compared to most desktops.

I ended up going this route because Raspberry Pis are so hard to come by these days. A Raspberry Pi 4B with 8GB of RAM will cost $75 and then you'll need to supply your own storage (MicroSD), buy a case if you want to protect it, and a USB cable and power adapter (though you probably have that sitting around). Between the Raspberry Pi and MicroSD card, it's basically $100 and way less powerful than a Core i5-6500T (which should be 3x faster) and you're using a MicroSD card rather than a nice PCIe flash drive. Plus, if you want, you can load the micro/tiny machine up with 32GB of RAM for not that much ($40 to get it to 24GB, $70 to get it to 32GB) and you can make that decision in the future. Plus, for a lot of purposes, standard x64 hardware can be nice. Plus, you can actually get your hands on a micro/tiny form factor PC while I haven't seen Raspberry Pis in stock in a long time.

One could easily disagree, though. The operating costs are likely to be an additional $10-11/year and one could get a 1GB RAM Raspberry Pi for $35 (if they had them in stock) and get a small MicroSD and maybe only spend $40-45 instead of $120, but it feels like the utility of a $45 machine with 1GB RAM is a lot more limited and I'd (personally) rather spend the money on something I know I can use.