DS web browser
This works for the Nintendo DS Lite. It uses WiFi and the Opera browser to give you a decent web browser wherever you have a WiFi access point:
This is almost as good as having an iPhone and a lot cheaper ;-) I've always thought that the Nintendo DS was underrated as a portable computing system. It's too bad that Nintendo doesn't make software development easier for this platform or encourage other non-game applications of this terrific device.