Wikipedia
SoftICE is a kernel mode debugger for Microsoft 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. Because of its low-level capabilities, SoftICE is also popular as a software cracking tool.
Microsoft offers two kernel-mode debuggers, WinDbg and KD, for no charge. However, the full capabilities of WinDbg and KD are available only when two interlinked computers are used. SoftICE therefore is an exceptionally useful tool for difficult driver related development. The last released version was for Windows XP.
Older versions exist for DOS and compatible operating systems. SoftICE was originally produced by a company called NuMega, and was subsequently acquired by Compuware in 1997, which in turn sold the property to Micro Focus in 2009. Currently, Micro Focus owns the source code and patents, but is not actively maintaining SoftICE.