During EUROBIKE 2009, which took place in Germany earlier this month, Dahon unveiled their Pango helmet. Once FOLDED (!) the helmet takes up half the space of a regular one becoming insanely compact. Here's the video showing how easily it folds.
Then there is the much talked about helmet Fuseproject was commissioned by NYC's DOT to design. While there are helmets on the market that have large enough ventilation holes that allow to be U-Locked, Fuseproject's NYC Helmet design ensures that it can be with its design. Also, its polystyrene shell appears strong enough that it could be left out on the city streets and handle the elements.
Top two photos via Dahon & bottom via Fuseproject