The Tclkill package provides a cross-platform "kill" command.
See http://www.fpx.de/fp/Software/tclkill/
The motivation is to terminate background processes that were started with exec, which in this case returns a (list of) process identifier(s).
The discussion on the kill provides various references to packages that provide this functionality, e.g., TclX on Unix, TWAPI on Windows, and various external utilities, but neither is cross-platform.
This package includes a DLL for Windows, a shared library for Linux/x86, and on other platforms it does "exec kill $pid".