I live in a house where there's no airing cupboard and the radiator in the bathroom is next to the toilet. So I bought radiator hangers, one for each family member, and we keep our towels in our bedrooms on these. The ones I have are plastic coated wire frames and I bought mine from Argos. They're great as long as you don't have really tiny radiators. I also keep one towel for everyone for handwashing in the bathroom just hung over the bath edge.
I moved into a smallish house a couple of years ago. Reasonable size bathroom but with a tiny towel radiator about two foot wide and three foot high. The only other clear wall space is above the bath. There are four of us: me, DH and two DS. Lots of baths / shower taking. So the place (ie my banisters) is always covered in damp bath towels. What do other people do?! Any suggestions?