What software can I use to poll my audience during presentation?
The Software Carpentry instructors do lots of presentations where they need/expect rapid, interactive feedback. They (especially Greg Wilson) have lots of experience of different ways of doing this. Note that their current method, that works extremely well, is to use different coloured sticky notes.

I haven't seen a technical solution that worked better; most are considerably worse. However, this is only practical for yes/no or multiple choice polls.