Building sophisticated educational tools out of cheap parts, Johnny Lee demos his cool Wii Remote hacks, which turn the $40 video game controller into a digital whiteboard, a touchscreen and a head-mounted 3-D viewer.

Athlete, actor and activist Aimee Mullins talks about her prosthetic legs — she’s got a dozen amazing pairs — and the superpowers they grant her: speed, beauty, an extra 6 inches of height … Quite simply, she redefines what the body can be.

Bill Gates hopes to solve some of the world’s biggest problems using a new kind of philanthropy. In a passionate and, yes, funny 18 minutes, he asks us to consider two big questions and how we might answer them. (And see the Q&A on the TED Blog.)

With the same humor and humanity he exuded in An Inconvenient Truth, Al Gore spells out 15 ways that individuals can address climate change immediately, from buying a hybrid to inventing a new, hotter “brand name” for global warming.

In the year leading up to this talk, the web tool Twitter exploded in size (up 10x during 2008 alone). Co-founder Evan Williams reveals that many of the ideas driving that growth came from unexpected uses invented by the users themselves.