Windows only: KillProcess lets you kill single or multiple processes in one swoop, manually or through predefined "kill lists". It's a fully flushed out utility that even lets you terminate protected Microsoft system processes.
KillProcess displays the memory usage of each process and lets you terminate processes effectively. Unlike previously mentioned Task Killer, and the robust Process Explorer, KillProcess has customisable "kill lists". Each kill list, a list of processes which you can add and edit to your liking, can be handled differently by KillProcess. For instance, the program can be set to scan certain kill lists during various time intervals to terminate on sight. KillProcess is a freeware program for Windows only.