1) Reuse - Reusing stuff is one of the simplest ways to go green for free. Need rags? Don't buy them from Target, cut up old threadbare shirts or sheets, these make perfect rags. Storage containers? Reuse old grocery bags, beer crates, produce boxes!
2) Recycle - Take your old bottles and cans to be turned to cash in bottle deposit states, something that I think should be expanded. In those states you will make 5-10 pennies per can. Even if you don't live in one of those states, recycling require minimal effort, especially in single stream recycling communities.
3) Borrow - Before you go out and buy something new, especially a book or media, ask your social network if they have it available for you to borrow. Maybe you can repay them buy letting them borrow one of the many things you already own, or trade another service such as cooking or watching their children. Libraries are a great resource, not only for dusty old books, but DVDs and Blu-Rays are available in most towns.
4) Re-gift - Giving green gifts is an extremely effective way of saving money. You can bake, cook, build or create online tons of gifts that will be appreciated. You can also let your extended family know that you would prefer they not purchase you any items for upcoming events. Let them know your philosophy about buying frivolous things and the impact it has on the environment. They might think you a little eccentric, but will respect you. If you have something that you received as a gift, don't feel bad to re-gift it to someone else, just make sure you don't give it back to the person who gave it to you!
5) Exercise outside - Don't get me wrong, I love to exercise in the gym, but that $30 a month that Gold's gym takes from you each month really adds up. Not only that, but they keep the lights, TVs, and legion of machines on 24 hours a day at many gyms, no need to support that! Get outside, take a hike, run, walk, play. It's free!
I hope I can come up with more cost-free tips to go green, but feel free to check out my list of 20 Effective Ways to Go Green that Make a Difference.