A Device driver that doesn't come with source. BinaryDrivers are often used by companies that think that giving out the source to their driver gives away too much about how their hardware works. Of course because they are compiled separately to the kernel, they tend to be bound to specific versions of the kernel, and you have to wait for new versions of your driver to be available before you can upgrade.

All in all, BinaryDrivers suck. :)

In the Linux world, one well-known example is nVidia's binary NvidiaDriver? Kernel Module for their accelerated graphics cards. While it is understandable that they don't want their competitors benefitting from their (purportedly highly optimised) graphics driver routines, it also means the Linux community could not fix any of the inevitable (and often system stability jeopardizing) bugs in these drivers. Indeed, issues relating to ClosedSource drivers were so common that the Linux kernel folks added a "taint" flag that indicates whether any of the loaded Kernel Modules are released under a non-free license (which BinaryDrivers necessarily are). They refuse to even look at any issues encountered while running a tainted kernel.