Two years after the first low-cost radio-frequency audience response system using clickers was introduced for college classrooms, at least six different systems are on the market. Their features and user-friendliness are evolving rapidly, driven by competition and improving technology. The proliferation of different systems is putting pressure on universities to standardize or otherwise limit the number of different clickers a student is expected to acquire. To facilitate that choice, the strengths and weaknesses of six systems (instruction Classroom Performance System, Qwizdom, TurningPoint, Interwrite PRS, iClicker, and H-ITT) are compared, and the factors that should be considered in making a selection are discussed. In our opinion, the selection of a clicker system should be driven by the faculty, although students and the relevant teaching and technology support units of the university must also participate in the dialogue. Given the pace of development, it is also wise to reconsider the choice of a clicker system at regular intervals. (Contains 6 figures and 1 table.)