Obsidian is a so called non-intrusive debugger for NT systems. It does not use the Win32 Debugging API for debugging purposes therefore Obsidian makes it possible to attach and detach from a running process transparently - normally a process will die along with the debugger. Check out
Obsidian: deneke.biz for further details.
Related work:
Gemini Lite: A Non-intrusive Debugger for Windows NT