Watching videos on the screen of the iPod nano is a pretty cool feature. However, if it's not a cartoon or three minute video podcast things can get a little rough on your eyeballs. If you don't feel like shelling out $100 for Apple's universal dock and AV cables, and you happen to have a slide viewer laying about, you're in luck.
Ingenious Brit and YouTube user markrirwin has developed a solution called the NanoScope. Taking an old 35mm film slide viewer and adding external speakers, he created a cinema-in-a-box magnifier that makes those tiny videos a reasonable size on its 4 inch screen.
While you'd look (and sound) kind of silly carrying this around with you on public transit, the device's wide base and fairly decent viewing angle make for good horizontal viewing.