I've already ranted on a bit about how I like Thunderbird's vertical preview pane (and Outlook 2003's, which I expect to be able to enjoy soon), but finding out that you can alter NetNewsWire to have that layout was a most welcome coincidence (I just installed the latest beta to try out, and was extremely impressed by the speed and polish of it).
After having done this sucessfully to my own copy of NetNewsWire, I started wondering. My NeXTSTEP days had shown me that .nib manipulation was extremely powerful, and that the design model made for almost complete independence between code and layout resources.
So I decided to hack my own vertical preview e-mail client: I made a copy of Mail.app from the /Applications folder and, after some poking around to find the right file, edited the Mail.app/Contents/Resources/English.lproj/MessageViewer.nib to achieve the same effect and change Mail.app's preview pane to a vertical orientation.
(I'm not going to publish more details on how I achieved it - asides from the pretty obvious .nib name above - since Apple is not likely to appreciate this.)
So far my copy of Mail.app works without ill effects (and does feel a lot like Thunderbird), but of course your mileage may vary. I am not responsible if you lose all your mail, if your cat explodes, or if you damage your Mac OS X install.
Remember, I edited a copy. Besides, I always run Mac OS X as an unprivileged user, since being able to throw system folders into the trash is not my idea of "ease of use".
Flash Adventure Game
Via Russel Beattie, a link to a brilliant idea (chess is known world-wide, is turn-based, time-independent, and board state can be encoded in a few bytes, so theoretically you could even swap SMS directly with another player's phone).