Been working on a project which intersects with a lot of the functionality of Darwin's CoreFoundation. Namely, a TCP/IP server application that does a lot with XML. The interesting part is that the market for this program is windows machines, and eventually some unix machines.
CoreFoundation works remarkably well on Windows, it took a little patching to get the RunLoop and CFSocket code up to speed, but now it seems to be working well, and makes for some highly portable application code. Already I've run into a number of areas where my life is significantly easier because I have some CoreFoundation data-types to back me up.
I'm leaving civilization for a couple weeks, but when I return I have to organize my patches, make sure they're sane, and then send them back for integration. Most of the stuff outside of CFSocket will be good fixes for Darwin (mostly code laziness that gcc doesn't freak out over), and the CFSocket stuff is mostly windows specific fixes.