The black leather couches and velvet curtains of Noir (1 Bennett St., Cambridge, 617/661-8010, 4 p.m.–2 a.m. daily, www.noir-bar.com ) could easily serve as the backdrop for an illicit affair. Located in Harvard Square’s  Charles Hotel, the lounge does its best to stir up intrigue with cocktails named for classic film noir movies.
The more rambunctious Grendel’s Den (89 Winthrop St., Cambridge, 617/491-1160, 11:30 a.m.–11 p.m. daily, www.grendelsden.com ) is a subterraneous lair where surly wait staff serve pints and finger food to a heavily Harvard-saturated crowd.
In Central Square , the Plough & Stars (912 Massachusetts Ave., Cambridge, 617/576-0032, 11:30 a.m.–12 a.m. daily, www.ploughandstars.com ) is an Irish bar with a working-class feel. It features an eclectic mix of live music and European soccer matches on the telly.
The New York–cool Middlesex Lounge (315 Massachusetts Ave., Cambridge, 617/868-6739, 11:30 a.m.–1 a.m. Mon.–Wed.; 11:30 a.m.–2 a.m. Thurs.–Fri.; 5 p.m.–2 a.m. Sat., www.middlesexlounge.com ) puts its couches on wheels, the better to convert from techno lounge to dance floor with the shake of a booty. The zen minimalist vibe attracts a cross-section of Cambridge  pseudo-literati.
At MIT’s  neighborhood bar, Miracle of Science Bar + Grill (321 Massachusetts Ave., Cambridge, 617/868-2866, 7 a.m.–1 a.m. Mon.–Fri.; 9 a.m.–1 a.m. Sat.–Sun.; www.miracleofscience.us ), the menu is modeled after the Periodic Table of Elements. Amorous nerds use their laptops to send instant messages across the room.
An Irish bar on steroids, The Burren (247 Elm St., Somerville, 617/776-6896, 11:30 a.m.–12 a.m. Mon.–Fri.; 11 a.m.–1 a.m. Sat.–Sun.; www.burren.com ) nevertheless manages to keep it real with Irish seisiuns and step-dancing on a stage in the back room. On other nights, patrons drink their stout while listening to open-mic poetry.