Since I can't afford equipment, I've been sticking to old fashioned workouts (at least trying to) because they work. But I have to admit those crunches suck. But these days there are all sorts of exercises created to match different people.
Lately, I've been trying these upside down abdominal exercises from Jillian Michaels. I definitely feel my muscles working and I like it since it's all done in a standing position. I don't particularly care for this woman, but I liked the concept and watched this video
Upside Down Abdominal Workout Video.
Also, have you thought of doing crunches on those big exercise balls? I found that the bounce of the ball was easy of my back, but harder on my abs because I had to use the ab muscles to keep the ball from rolling out from under me. Sometimes you can find these balls on sale at places like Big 5. I got my like, $40 ball for only $12.
Anyway, my point is that it's about experimenting and there are lots different options if a little exploring is done.