I found this during research and thought I'd share. It could be better, but it's a one-person project labor of love, so I'm pretty forgiving.
For new-ish parts, PCPartPicker.com is still the single best resource for this info, but their info isn't always entirely accurate. For a given board I was researching with USB3.0, PCPartPicker listed it as having USB3.1Gen1 support. For compatibility its built-in checking is pretty good for what it does, but if your use case demands specific I/O ports/functionality, especially dicey things like Thunderbolt over USB-C, always discover the ground reality for yourself from first-party sources, such as the OEM/vendor.
For new-ish parts, PCPartPicker.com is still the single best resource for this info, but their info isn't always entirely accurate. For a given board I was researching with USB3.0, PCPartPicker listed it as having USB3.1Gen1 support. For compatibility its built-in checking is pretty good for what it does, but if your use case demands specific I/O ports/functionality, especially dicey things like Thunderbolt over USB-C, always discover the ground reality for yourself from first-party sources, such as the OEM/vendor.
https://pcpartpicker.com/products/motherboard/
Here's some other motherboard research links I found:
https://acme.com/build_a_pc/boardfinder/
http://www.motherboards.org/mobot/stats.html