We're after a driver for an Intel PRO/1000 GBE network adapter which runs entirely in user mode - i.e. will not cause any user/kernel context switches in normal use. We need to be able to send and receive small UDP packets at a far greater rate than we're able to using the driver in the kernel, and eliminating the context switching and kernel protocol stack looks like the way forward. The result can be delivered on Linux, FreeBSD or Windows - we're not fussy..
Source for Intel's driver is available from their website, and ought to provide a solid starting point.