From the entry on the NCDD resource center:
Keypads are audience response devices that look like little calculators or remote controls. They are used in group meetings or events to collect audience responses or opinions, allowing audience members to interact with presentations, give feedback, and become more engaged in large-group settings. Keypads are often used as an added element in large-group deliberation and dialogue events.
This resource lists and links to experts in keypad response system as well as mobile device response systems, as recommended by NCDD members in April 2010.