I was on my new MacBook Pro the other day, working on some database stuff when I accidentally hit ⌘+comma in a Finder window. This keyboard sequence brings up the software preferences for whatever program you have activated. I was a bit surprised to see the Finder Preferences window pop up because I hadn't seen it in over a year. Since I was on a new machine I clicked through the various tabs in the window and noticed the Show all filename extensions in the Advanced tab. I'd never seen this setting before so I turned it on. And it did exactly what it says it does, makes all extensions of every file show up in all Finder windows.
I like this setting as I'm always editing file properties (⌘+i) that don't show an extension to force the behavior. The only downside to this setting - if you can call it that - is apps in the Applications Finder window show a .app extension. That's probably the only file type I don't want to show an extension.
Here's a screenshot of the Advanced tab settings in the Finder preferences window. Maybe someone else will be as excited to find this feature as I was. Incidentally, I thought this might've been a feature added in Snow Leopard. Sadly (sad that I didn't know), I checked an OS X 10.5 machine and saw the same setting.