Those old lawn mower blades are excelent metal for working into things. The majority of mower blades are made from 1084 high carbon steel, or 5160 carbon/alloy steel. They can be used to make some very good knives, scrapers, chisels, and most things for cutting. I also use them for making flint strikers. I pick them up whenever I get the chance - for free of course.
Hope this helps.
Mikey - that grumpy ol' German blacksmith out in the Hinterlands