This shortcut is genius. In TextMate for example, you can type the first few letters of a project and open it near instantly if it's still in your recently opened list. It's one of my biggest productivity differentiators in OS X. You should all try it.

I think it's enabled by default on Snow Leopard (not on Leopard). You'll find it under Setting > Keyboard > Keyboard Shortcuts > Application Shortcuts > Show Help menu.