All these projects, software solution or service providers use iPXE in some way.
Netcup (used as netboot system on vservers for installations and to manage the rescue system)
Hetzner (used integrated ROM builds in EX* server series for boot into rescue system and for internal deployment of installations like vSphere)
Rackspace (use it as part of their provisioning infrastructure)
VirtualBox (included as the default netboot option ROM if you don't install the non-free extension pack)
Joyent (used as part of the SmartDataCenter product)
Walmart (used as part of their infrastructure)