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.

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