I participated in my first fabric swap on flickr this past spring and I've been hooked every since. We swapped 5 inch charms of polka dot fabrics. I used mine to make this polka dot pinwheels quilt top.
If you don't find the swap you want, you can always host your own. I was looking for an I spy swap but there were none going on, so I jumped in and hosted one in March. There's some great instructions on how to host a swap here.