A DDK is a Driver Development Kit. It's a more specific form of an SDK, usually for Kernel mode development on a non-open source system. Microsoft is the only vendor off the top of my head that produces (or more specifically referrs to it as) a DDK. (msg me or add-on if I am incorrect). Apple does produce a Kernel module loading system for OS X, and i think that driver developement under earlier Mac OS's just used a more cryptic set of Extension APIs.

With the MS DDK, you get a build environment (which still requires the cl compiler), documentation, and how-to's for producing drivers for each major OS release. There are also HAL toolkits for NT (if you are brave enough to write your own HAL). It's a hairy process and requires a serial cable for two machine debugging using windbg.

Apple's IO Toolkit, and Kernel toolkit also require two machine debugging using arp over ethernet and remote gdb.