There are several web-based "audience response system" packages available. Poll Everywhere offers some pretty nice features including various input methods (SMS, Smartphone or desktop/web-based voting), polling/output views (e.g. hidden polling results during voting and customized output graphs) and Powerpoint integration.

Following is a demo of the system that you can try:

Question: Which device would you prefer to use as an audience response system?
Possible answers: Cell Phone, Proprietary Clicker, Web Browser

To answer via SMS:
- Cell Phone (text "CAST 19892" to 41411)
- Proprietary Clicker (text "CAST 19893" to 41411)
- Web Browser (text "CAST 19891" to 41411)

To answer via Smartphone/iPodTouch visit http://poll4.com:
- Cell Phone (enter "19892")
- Proprietary Clicker (enter "19893")
- Web Browser (enter "19891")

To answer via web browser on a computer.

The system also allows you to embed the poll into a page (along with the current results).