OpenStack is an open source solution for running a public or private cloud. Many of the OpenStack services use a MySQL database as a storage backend to store their own state. In addition the Red Dwarf OpenStack component provides a Database-as-a-Service solution, again by default based on MySQL. OpenStack is the largest open source cloud community, with Rackspace, HP, Dell, Red Hat, etc... as members.
The OpenStack manual recommends Galera replication to make MySQL instances highly available.