I've been using Movie Magic Screen Writer for a while now and I love it, I actually prefer it to Final Draft which I used to use. Less glitchy, very easy to use, gives you everything you need (has all the predicitve spelling for names, locations etc), saves to PDF and also has a neat funstion where a computer voice reads out your script for you. Fair enough its very simplistic and sounds like Stepehn hawking and his female counterpart chattering away but it helps in finding errors and judging the pace. Plus its cheaper.