One of Roppongi's more enduring expat hotspots, Havana Cafe serves good mojitos and some decent Tex-Mex grub, with a kitchen that stays open until 4am. The ground-floor standing bar is usually the livelier option, though those in search of a quiet meal can retreat to the basement instead. Best of all is the happy hour from 5pm-7pm, when you can get a gin & tonic or a Moscow mule for ¥200, wine and beer for ¥300, and a frozen margarita for ¥500. It'd be good value wherever you were – but in the middle of one of Tokyo's pricier nightlife districts, it's an absolute steal.
UPDATE: Havana Cafe is now also open for lunch.