This game can be run with medium to large sized groups and is a good one to teach the concept of trust.
Make a big circle and get everyone to put their right leg in towards the circle. You'll then need to get everyone to move in to shrink the size of the circle. Keep moving everyone in until people are touching the person either side of them.
On the count of three, get everyone to put their hands on the shoulders of the person in front of them and SLOWLY sit down. People may be hesitant at first or may end up half squatting, half sitting, but done properly (and with trust!), everyone ends up sitting on everyone else's lap!
It's a good icebreaker - it involves laughter, trust, a shared achievement and breaks down barriers.