he way Brainify tries to set itself apart, however, is in its exclusivity, Mr. Goldberg said. Unlike general-interest sites like Delicious, Diigo.com, and Ma.gnolia.com, Brainify restricts membership to those with college e-mail addresses. And rather than link to fried-chicken recipes or the latest YouTube hit, users are likelier to bookmark animated illustrations of particle physics or explications of John Donne’s poetry.