Advanced Process Terminator and PSKill
Advanced Process Terminator is a small (42 kb) free tool from DiamondCS (of TDS fame). It has a neat little user interface which displays running processes along with information like PID, file location etc.
But the real fun lies in the number of ways you can kill a process. It has 9 methods to kill a process. By this, it can also kill "toughest" running processes which normal Windows Task Manager can not kill! It can also suspend and resume processes.
Another process killer tool is PSKill from Sysinternals (of Process Explorer fame). This is a command line tool and is very easy to use. It can also kill processes like Winlogon.exe which Windows Task Manager is not able to kill.