I always use the Eco range stuff which I find is great for keeping things soft and fresh. I can't for the life of me remember what the second half of the name is, it's Eco something, about 6 letters long altogether, and the 'Eco' part is green and the last 3 letters are in blue.
The best thing I use on my towels (and in fact now all of my washing) are the balls which you put in the washing machine drum with your washing instead of any other powder or liquid stuff. I can't find the branded ones anywhere on the net but these ones on ebay look exactly the same and are great value considering I paid �11 for one ball and a stain remover stick a few years ago. They last for ages too :)
Good luck, this should save you buying a new tumble dryer!!!