i birthed on my knees, over the end of the bed.. i think the hwole gravirt thing helped LOT. (
Ditto. Three of my deliveries have been on my knees leaning over the raised end of the bed and cushions. They've been the easiest, by far.

Dd1 I was almost flat on my back, and that wasn't much fun.
Dd2, I was sitting up with my hands on my knees. Not too bad.
With dd3, I was labouring on all fours to get rid of a cervical lip, when all of a sudden she was born, and I realised how comfortable it was in that position.
With dd4, it was a bit of a rush - I was in the bathroom and felt the baby start moving down and out so dh called for the midwives who got me on the bed and got me to lay down to check where the baby was at - at which point she started to be born and I couldn't move - flat on my back HURT.
With dd5, I was up on my knees as soon as I realised we were close, as I knew how comfortable that would be.
With Tom, the OB asked me how I preferred to deliver, and I said on my hands and knees, so they cranked up the bed head. Then when I hit "transition" (ha) I decided I really didn't want to be like that, but it was too late because he was shooting out and bang, there he was.

My sister squatted on the floor to have her last baby, as she was having a bit of a difficult time getting him out after over an hour of pushing, and it worked well for her.

If you move around a lot during labour, you'll get a good feel for the positions that are more comfortable.