I've used Quitling, which I still use, and it works well. It can only kill one thing at a time. I also use Quit Anything, but it seems to have stopped further development. It occasionally gets AppleScript errors and is somewhat unstable. I does kill as many things as you want. This is an advantage over Quitling.

I've pretty much switched over to QuitsApps because it works first time, every time. I use Retrospect for backups and it takes over the computer when it runs on my old G4. I use QuitsApps to kill everything before Retrospect runs on the days that it runs. I made a donation and will pay for registration if it becomes shareware. It is such a great time saver.