The Apache Software Foundation (you might have heard of them ;) runs their servers on FreeBSD.
NetFlix runs some (all?) of their servers on FreeBSD.
Google uses FreeBSD for their network search applicance (for corporate intranets or whatever). They also just announced their funding of the Capsicum security framework project for FreeBSD (and later, a port to Linux).
The Weather Channel uses FreeBSD to power their network appliances that render the forecasts for local cable networks.
NetApp and Juniper Networks use FreeBSD for their networking gear.
There are more examples here if you want to read them:
http://www.freebsdfoundation.org/testimonials
The FreeBSD Foundation also has a list of donors:
http://www.freebsdfoundation.org/donate/sponsors
The FreeBSD forums are reasonably active and very friendly, you should check them out if you want to learn more (or have more questions about *BSDs in general):
http://forums.freebsd.org/
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EDIT: I forgot one; VeriSign apparently uses FreeBSD enough that they just started their own BSD-related conference:
http://lists.freebsd.org/pipermail/freebsd-questions/2013-Ap...