We've always felt that pico projectors are a little uninspiring, but the Pop Video iPhone projector from Micron Technology displays higher-resolution than the rest, and it's small and cheap enough to be practical.
The 99g projector connects to your iPhone via a retractable 30-pin dock connector and displays video at the iPhone's full 960x540 resolution (similar projectors are usually trimmed to 640x360). The corresponding Pop Video app allows you to tweak the image. On a full charge, the little guy will run for two hours, which is roughly long enough to get your through a feature-length film. Obviously, it requires a pretty dark space to produce a decent picture, but for that $US99 price, it's a nifty little accessory. The Pop Video projector is available for pre-order now. [Micron Technology via CNET]