Running old games from Windows via DOSBox?
Running old games via scummvm?

Reading books using FBReader with a screen that is much better than the iPhone?

MaemoMapper, and using it with a variety of types of maps. When I recently went to Wales for the weekend, I had maps and satellite photography downloaded before I left so I didn't have to worry about signal along the coastal paths. As we were out walking the rain rolled in faster than expected, and I steered us onto the runways of a disused military airport so we could avoid the increasingly deep mud and get back to the car more quiickly.

FMTransmitter -- Sending the audio of YouTube videos through the car's stereo system without using a cable. Audio books for free, essentially, with all sorts of content available. No 3G data costs or dropped signal as I had downloaded these over WiFi at home using @Scene, an offline YouTube client.

Orrey -- used it last night to confirm that the bright thing the sky was indeed Mars, but that the other bright lights nearby were stars in specific constellations. This was with the Planet Compass function.

Streamed Spotify to my N900 using yaspot and qtify. A million song library for £10/month that doesn't take up any shelf space? Brilliant!

mwtube -- OK, not unique, but very handy for me. A dedicated and fast app showing me the status of the London Underground system that I commute on.