I used to use this program called "App switcher" but this one is better and free. It allows you to use the "command+tab" command to switch applications but also allows you to simply hit "Q" to mark an app to quit or "H" to hide it, etc. Easy to quit a bunch of apps quickly.
My brother is a big fan of GoMac in OS 9 and wanted to have the ability (in OS X) to auto hide apps except the one he's working on, this little program can do that too. In the LiteSwitch control panel, on the second pull down menu change it to "single application mode".