Student Response Network (SRN) is a powerful "virtual Clicker" student response system designed for use in school computer labs or with wireless laptop groups. It does away with the need for personal handheld "clicker" devices by providing a software-only solution for use with networked Windows desktops or laptops.
free version limits the number of users, but prices are very reasonable:
$49 one server/40 clients $99 unlimited servers/40 clients
Very easy to set up!
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