The two products i like best are composted cow manure and hay; both free and work every time. Eventually lime will be added to that list, as i have heard good results come from this simple natural ingredient.
My first thought was poop. Around here hay ain't free. Even spoiled hay is used by landscapers and roadside restoration. Lime shouldn't be added indiscriminately, only if your soil is too acidic.
The first thing to do is have your soil tested.
I used Netune's Harvest before and it is a really good product, although quite costly.
For the cheap route, Manure, cow, horse, chicken, rabbit, just be sure it's composted well before planting so you don't burn the plants. Leaves make a wonderful compost once decayed. Grass clippings, Hay as mentioned, wheat straw, and something I try to do to the garden once a year, plant a green manure. In the summer I will grow a crop or two of Buckwheat and turn it just before it goes to seed. In the fall to over winter I used annual ryegrass. Then turn it in the spring. The only issue I've ran into with that is getting the ground worked if we have a really wet spring. Usually I will do the rye in 20' strips so I have plenty of room for planting while I'm working up the rye.