While the NBA's collective bargaining agreement includes certain scenarios in which a team needs a player's approval to trade him, it's rare that an actual no-trade clause is negotiated into a deal. Heading into this summer, only Kobe Bryant and Dirk Nowitzki had official no-trade clauses in their contracts.
However, according to Mark Deeks of ShamSports, the number of no-trade provisions in the Association doubled this offseason. Deeks tweets that both Kevin Garnett and Tim Duncan have no-trade clauses in their new deals with the Celtics and Spurs, respectively. Both players inked three-year contracts this summer, so they can't be included in trades until those deals expire in the summer of 2015 (or 2014, for Duncan, if he declines his third-year player option).
Of course, it's unlikely that either Garnett or Duncan would have been in danger of being moved even without the no-trade provisions. Duncan has been a Spur for his entire career, while Garnett recently reiterated that he intends to retire as a Celtic.