Roadrunner Twice

Mac OS X ships with two QWERTY keyboard layouts meant for American-style keyboards: U.S. and U.S. Extended. U.S. can generate some accented and extended characters using the Option key, but is careful not to output anything not contained in the old-skool MacRoman character set; U.S. Extended can make a crazy amount of accented characters (including ō, Ǽ, and Ž), but most of them are only safe to use if you're in a Unicode document of some kind. (As in, they're usually fine, but it's going to be really confusing to you if they aren't.)

The OS also ships with a Dvorak layout (which I use) that can generate the exact same set of extended-ASCII characters as the U.S. layout. If you need any of the even-more-extended Unicode characters, though, you have to switch your keyboard and brain back into QWERTY mode, which blows.

So on the extremely off chance that you type Dvorak and occasionally find yourself needing to spell "shōnen" correctly, you are now covered, courtesy me and Ukelele.

(Confidential to boopsce: Finite-state automatons are crazy. I was actually able to get this done using only the "Swap Key..." command, so I didn't have to mess with the actual chains of dead keys, but the amount of stuff that was possible in there blows my little mind.)