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I wondered if there might be, but did you post the right link? I can't see anything about open source USB device implementations there.

Kate Temkin's Luna project is super cool, although I think it's still a WIP: https://github.com/greatscottgadgets/luna

It's a USB stack implemented as pure gateware - no hard/soft CPU needed, although I think you can hook one up if you want to.

The code linked there is all open, and has a USB device softcore. It's the most approachable FPGA USB implementation I've played with.

edit: Here is the actual USB core that's in there: https://github.com/im-tomu/valentyusb/