The only one I currently use is NoScript - very handy blocking tool.
I think the idea is so that any really common scripts such as google stuff will be deactivated in general and you can then activate scripts for a particular site to make it work.I can't see why NoScript is so popular! After hearing a lot about it I gave it a go.
It's annoying as hell! Having to unblock scripts every time you visit a 'new' website just to get the site to work properly gets very tedious, very quickly. Blocking JS globally on the off-chance that you stumble onto a dodgy site just seems like paranoia to me.