Single-Application mode in OS X

[*LTD*]

http://db.tidbits.com/article/10624

You can still do it.

 

[devburke]

Well you can also just cmd-option click the dock icon to hide all other apps. Or cmd-option-h. And you can hide/unhide apps by pressing h in cmd-tab mode, which works great if you want to hide/unhide a lot of apps at once.