SoftICE is a kernel mode debugger for DOS and Windows up to Windows XP. Crucially, it is designed to run underneath Windows such that the operating system is unaware of its presence. Unlike an application debugger, SoftICE is capable of suspending all operations in Windows when instructed. For driver debugging this is critical due to how hardware is accessed and the kernel of the operating system functions.
Details über Numega SoftIce: https://en.wikipedia.org/wiki/SoftICE