Do you mean libraries for backend languages, like Go and Java, or servers, like Nginx and Caddy? Because for Go there is the quic-go module[1], and since Caddy is written in Go, it already has (experimental) support for HTTP/3[2].

[1]: https://github.com/lucas-clemente/quic-go

[2]: https://caddyserver.com/docs/caddyfile/options#protocol

Here’s a list of implementations. There’s quite a few backend (either servers or server libraries.)

https://github.com/quicwg/base-drafts/wiki/Implementations

Hypercorn a Python ASGI server supports HTTP/3 built on aioquic which is a Python library supports QUIC.

(I'm the author of Hypercorn).

Envoy proxy has initial support using Google's Quiche. Docs and example configs are being worked on. I wrote up some h3 client and Envoy proxy testing this week [0].

0. https://dev.to/moderation/experiments-with-h3-clients-envoy-...

nginx has experimental QUIC support:

https://quic.nginx.org/

Depending on what you want to accomplish, an option is running your services in a cloud that supports terminating QUIC on their front-end load balancers. Google Cloud does, for example.