I took advantage of a Ryanair "free seats" offer before Christmas, myself, my partner and 2.5 year old went "free" (plus "taxes" etc) while our 6 month old who didn't occupy a seat cost £10. Other low costs allow infants to go free including easyJet.
Ryanair charge you for the privelige of carrying your infant on your lap, but don't allow baby seats on board. I am concerned at the effectiveness of these lap belts - again I'd be interested in findings from research and what other airlines do. On BA's web-site they talk about specially designed car-seat style seats for their aircraft anyone know anymore and why they aren't more common?