Life As Mom there is a feature called Frugal Friday where you can share your money-saving tips.
One thing that may stop you from exercising is the cost. Yes it may be pricey to join a gym- especially if you have to sign a year-long contract. If you look for a no-frills kind of a gym rather than a fancy health club, it may be quite a bit cheaper and you may be able to pay month to month. Also what ever sales pitch they try to sell you on, ask for a better deal- it never hurts to ask!
One very free way to get some exercise is of course to get out and walk. If you have bad weather there is always the mall- often they open early just for walkers (watch out for those seniors they'll run you over). If you have stairs in your home or at work you have your very own stair climber- no cost! You can often find a cheap bicycle or other exercise equipment at a garage sale or on Craig's List.
Another free option is the library. You can find diet and exercise books and DVD's. I get just about all my books from the library- I rarely buy books but when I do I get them used from Half.com. One more free option, if you have cable TV, check if your cable offers free on-demand videos. Mine offers The Biggest Loser workouts and many others for free.
Here's some free walking resources from the American Heart Association.