I used to buy boxed 4-in-1 lawn feed for about a tenner a go. I don't have many weeds, so it was mainly for killing moss. Then I discovered you can get a lifetime's (or at least a good few years') supply of ferrous sulphate for a few quid, and kill the moss very effectively with that.
Being slightly excited with this discovery, I was just wondering if anyone knows of any other chemicals that can be used around the garden/house, to replace other commercial products? I got some soda crystals to clean the washing machine with, and they seen to do a pretty good job of cleaning clothes, too.