The articles presented here demonstrate some interesting undocumented functionality and provide several ideas about using it.
Making your first Windows driver - complete walkthrough  This is a complete walkthrough on creating and debugging your first Windows driver using VisualDDK. It describes the entire process: from installing the required tools to stepping through your driver code in Visual Studio.
VMWare internals - GuestRPC

The GuestRPC mechanism provides an efficient way of exchanging messages between a VMWare host and a guest machine. GuestRPC relies on VMWare backdoor technology (an emulated hardware port for sending requests from guest machine) and allows executing arbitrary handlers inside the VMWARE-VMX.EXE process from the guest machine. This mechanism is used to transfer kernel debugging data in a faster way than a simulated COM port. The article describes how to use the GuestRPC mechanism using the KDVMWare implementation as an example.

KD extension DLLs & KDCOM protocol

All Windows versions from NT4 to Windows Vista support debugging via a COM port. In that mode a part of the operating system sends and receives data via the COM port using a special protocol called KDCOM. Understanding this protocol was one of the key points in KDVMWare project, as it allowed creating/parsing KDCOM packets on the host side, passing structured packet data directly to the guest OS. The article describes the KDCOM protocol in detail.

Any questions about the articles can be asked via the e-mail link on the main page.