Commercial

Cammy
Has about 10 services in production at thousands of requests per second.
Verizon
Uses http4s extensively in its internal services and open source projects.
看录取 Kanluqu
College application resources for Chinese high school students, built entirely upon the Typelevel stack.

Open Source

CouchDB-Scala
a purely functional Scala client for CouchDB
fleet-buddy
Eve Online fleet buddy based on the CREST API
http4s-demo
an open source demo app built with http4s
httpize
a httpbin built with http4s (source)
rho (ρ)
a self-documenting HTTP server DSL built upon http4s
0x7e.xyz
A simple link-shortener
http4sbin
A httpbin clone built with http4s