Near the Accademia, Il Piccolo Mondo, Calle Contarini Corfu, Dorsoduro 1056A (tel. 041-5200371), is open during the day but comes alive with dance music at night. The crowd is often young. It's open daily 10pm to 4am, but the action actually doesn't begin until after midnight. Cover, including the first drink, is 7€ Thursday and Friday, and 10€ Saturday.
Martini Scala Club, Calle Cafetier 1007, San Marco 1980 (tel. 041-52340027), is an elegant restaurant with a piano bar (and a kitchen that stays open late). The piano bar gets going after 10pm and stays open until 3am (it's closed Tues-Wed at lunch).
Early every evening except Wednesday, Paradiso Perduto, Fondamenta della Misericordia, Cannaregio 2540 (tel. 041-720581), functions as a likable tavern, serving well-prepared and reasonably priced platters of seafood Thursday to Monday 7 to 11pm, and Sunday noon to 3pm. After 11pm, a mix of soft recorded music and live piano music creates a backdrop for animated conversation between the neighborhood crowd and visitors. The chitchat continues until at least 2am.
Casanova Music Cafe, Lista di Spagna, Cannaregio 158A (tel. 041-5347479), is a claustrophobic dance club. It's not the world's greatest, but it is very popular and a good pickup bar for straights. The music is mostly recorded, ranging from alternative to Latin salsa. Come around 11pm, when the dancing begins in earnest. There's a cover of 10€ after 10pm.