A just-released app for the iPhone and iPod touch is LCD Clock from Japanese developer forYou inc. The app, called LCD Clock [App Store link] has several functions, including display of holidays (U.S. and Japanese), 12 or 24 hour time display, a choice of 4 colors for the clock and calendar, support for landscape as well as portrait orientation, and the ability to disable auto-lock so the display stays running. Be sure to activate that only when then iPhone is charging or you'll eat through your battery quickly. I had trouble finding the settings menu, and it turns out they are not in the app, but in the main iPhone settings menu. You can change colors and set the alarm from within the app.
I didn't find the screen displays particularly striking. I liked Touch Flip Clock [App Store link] a bit better for a more interesting design, but the on-screen controls are in Japanese. (edit- it's actually Korean. I regret the error). For something a bit more traditional there is an old style clock in the AA01 Led Time [App Store link] app.
My favorite clocks for the iPhone are apps that emulate the look of the old Nixie Tube units. Here's one, Nixie Clock A, [App Store link] that looks nice and supports an alarm.
All the clocks discussed are US$0.99 which seems to be the going rate, although there are some free ones [App Store link] around. Improvements I'd like to see in the clocks I've mentioned would be the ability to choose a music track for an alarm, just not the standard rings and buzzers. There are lots and lots of clock/alarms for the iPhone and iPod touch. It's really just a matter of finding one that suits your needs and personality.
If you are looking for an alarm clock app for your mac, TUAW has you covered as well, with an extensive review from Mike Schramm of what is out there.