Well it isn't free,but honestly Wal-Mart is your best friend. I have a VERY limited income, and I am home on disability due mainly to mental problems, so I have less than 700 a month coming in. Resistance bands are relatively cheap and you can take them with you, and that can substitute for free weights. The free weights there actually don't cost too much, but resistance bands are great to start out with. Or you can try cans of soup or cans of vegetables, and work your way up.
I second walking. That is always free. If you have a netflix subscription, they have tons of exercise videos on instant streaming. I have a ton saved on my queue (and I know a lot of people dislike netfix now, but I love netflix instant) and have done a few.