Rare indeed are nights when it is warm enough to go dining out in San Francisco alfresco, but when they do arrive, MarketBar, an American brasserie, achieves its full glory. Outdoor tables facing the Embarcadero offer excellent people-watching, especially at lunch, on sun-drenched weekends, and during happy hour, when the Ferry Building Marketplace attracts big crowds. While the California-style fare is average at best, the wine list, handsome bar, homely vintage feel, and prime location make it all worthwhile. On Saturdays, when the Farmers Market is in full swing, the atmosphere is positively gay.