Simone,
There where a few, one of which was discussed on the Handlheld Learning stand - for my sins I've forgotten their name (shame on me) but Graham et al., will fill the gaps.
Interms of a PRS - there is also Turning Point
http://www.turningtechnologies.com/ - this has PDA software (pocket pc, and palm on the way). I thought this looked really good as you can include free text answers, it works very well with powerpoint, and for those who need to pull results to central databases, and import questions there are some opportunities.
I hope that helps ....
Cheers
Andy