This is the way online radio was always supposed to work. Pandora allows users to select what artists they like, or even specific songs, and build their own radio stations. While that's not all that revolutionary, the site's technology employs data from Pandora's Music Genome Project, which theoretically helps listeners discover new musicians they are likely to love, using years of research. The result is said to yield far more useful info for selecting songs based on listeners' tastes (if someone tends to like heavy base guitar riffs, for example, they'll hear more songs that contain them). Advertising-wise, Pandora's just getting started, and its user base is relatively small, but given the level of user input, the behavioral targeting opportunities appear vast.—M.S.