I saw some wool balls that were supposed to cut at least 25% off the drying time if you put them in the dryer to roll around with your clothes. The tutorial said to make a yarn ball and then felt it.
You need a yarn that will felt, it will usually say it can be felted on the label. If it doesn't then you can assume 100% wool yarn will. (Another tutorial I found didn't use yarn at all, but used wool batting. I'll get some and try it, then post the results here.) The wool yarn I found was a bit pricey for me to make several balls out of it at $5 a skein, so I dug around in my craft stash for some wool remnants I'd used for some needle felting. The bits I had left over were just some matts and little snips that were left over after wool was carded, and though I'd been able to use a bit here and there for needle felting, I was about to toss it because I thought it wasn't any good. It turned out to be perfect for the core of a dryer ball!
The snippits of wool wouldn't roll so I just matted it together into the toe of a nylon sock and tied it off. You can hand felt it like this, or do it the easy way. I plopped them into a pillowcase and tied it shut, then ran it through a load of laundry. I wasn't sure it would felt well since some of the bits were so short, but it held together well enough that I could move on to the next step. The picture shows the raw wool, the nylon sock that has gone through the wash (the fuzzies on the sock are good, they mean the fibers have worked their way out and the inside is probably nicely felted.) Just peel away the sock and you have something like the felted balls on the right.
I took my wool yarn and wrapped it tightly around the felted core. I only wrapped enough around it to cover the felted ball, then tied off the yarn. This way I got the yarn to last for nine balls.
I put the wrapped balls back into the nylon and ran it through another load of laundry. I wasn't sure if you were supposed to take it through the drying as well, so I did, and it seemed fine.
The yarn felted to itself and will continue to felt more as it is used in the dryer so as long as it felted enough to hold together for now it will be fine.
Most people use four or six balls per load, I figure with nine, I have enough to alternate between loads, and have a few to spare for my little buddy, who found them irresistible. I might just have to make a few just for him, they are nice and lightweight and wouldn't do much damage if thrown.
If you'd like to see a tutorial on making these using wool yarn, try this one at Good Mamma.
After doing several loads of laundry using the dryer balls, I'm pretty impressed. My loads are done in 30-40 minutes, where I'd previously set it at 50-60 minutes. Maybe I was overdrying it before, it would be nice to test these in a dryer that shuts off when the clothes are dry. I had two balls unravel a bit, but I think it was my fault for not tying the yarn off well, and it was easy to tie them off again.
Another little tip: Put a few safety pins on two of the balls to cut down on static.