Fabric softeners, which can trigger allergy and asthma attacks, can be replaced with white vinegar. This substitution is not only green, it's frugal. I've never been a fan of fabric softener because I've never cared for the strong smell plus fabric softeners decrease the absorbency of towels. About 1/2 cup of white vinegar should do the job.
Of course the most sustainable way to dry laundry - and to prevent static cling - is to air dry it. However, not everyone has the time or space to hang out the laundry. And if you live in a humid environment, line drying heavy things like towels and jeans is not always ideal because you can end up with items smelling of mildew.
If you do need to use the dryer, static can be a problem. I've discovered two options for reducing this static. (Disclaimer: I have not yet tried these methods but have read a variety of reviews recommending them and plan to try them in the future.) One way is to use aluminum balls made out of...aluminum foil! Ehow has very easy instructions here. Once the balls are no longer effective, the foil can be recycled. In fact, the balls can be made from used aluminum foil that has been carefully washed and dried after use in the kitchen. Victoria of Green Idea Reviews gave this method a thumbs up.
The other green way to eliminate static is to use felted wool dryer balls. These balls can be made with a combination of wool yarn, wool roving, and/or wool fabric scraps. Ehow and Crunchy Betty both have good tutorials. (Crunchy Betty even recycled a moth-eaten thrift-store find in her dryer balls.) If you're not feeling crafty, you can buy hand-made balls on etsy. Just do a search for "wool dryer balls."
If you're not yet convinced to give up your favorite laundry detergent, fabric softener, or dryer sheets, check out the Environmental Working Group's 2012 Guide to Healthy Cleaning, where cleaners are given a grade from "A" to "F." I was surprised at the low grades given to products that I considered "green" and by how few products scored an "A." Making your own products ensures you know exactly what is going into your laundry - and into the environment.