The Firefox browser has a great little feature that warns you when you are closing a browser window with multiple open tabs. The next major version of Safari (shipping with 10.5, Leopard) will have this option too, but that’s not much help for those of us using Safari right now.
TabStop brings this feature to Safari, now. It not only warns you when you are closing a window with multiple tabs, but it also warns you if you try to quit Safari with multiple windows open… for those times when you accidentally hit cmd+q, instead of cmd+w.
via FreeMacWare