Eric Zitzewitz now out of Silicon Valley and San Francisco’s rich and remarkable local clusters based on industry niches, skills, school ties, traffic patterns, ethnic groups and even weekend sports teams —ALAS.

Hardware companies have clustered around industry leaders in Silicon Valley, while Internet advertising and design has taken root in San Francisco.

[...] The [Silicon] Valley residents see themselves as true entrepreneurs, entirely focused and dedicated. Marc Andreessen, 36, the co-founder of three companies in the Valley (Netscape, Opsware and Ning), concedes that San Francisco is generating more start-ups these days. “But in general, the nerds with minimal social lives like me are well down in the Valley, and the cool kids with the trendy glasses and Prada shoes who like to go to parties are in San Francisco,” Mr. Andreessen said in an e-mail message. “You can guess who has the leg up in building companies.”