k8temp is a tool for reading the on-die temperature sensors on AMD K8 processors (Athlon 64's and Opterons) on FreeBSD, and probably the other BSD's provided they have a similar /dev/pci.
And yes, unlike my other projects, I'm actually releasing this one, since it's nice and small and mostly works. I'm surprised nobody's done it before, actually; it's just poking at a PCI device register.
(Update, 2007-10-24: now available as a FreeBSD port)