Uuuf, where to start? You can get (amongst others)
Arrakis (terrible, terrible)
ya.com (good)
Jazztell
Orange
Linea2
BUT, they are use the underlying infrastructure of Telefonica, they are just resellers.
A good option (if you can) is to try and get cable with someone like ONO. TV, Phone and Internet at a far superior quality than Telefonica ever will be able to manage without ripping up Madrid first.
I've had ya.com in the past and they were good. I only switched to Telefonica because my company now pays the internet bill so they choose the supplier :-)
One thing to bear in mind though, if you have anyone other than Telefonica, any problems you have will take longer to resolve if it is a problem unrelated to your computer. The reason is that you'll log the call with the reseller, they will try to help you out, but when they decide it is a problem with the line then they phone Telefonica and you are left to the mercy of Telefonica. Whereas, if you choose Telefonica then you are talking directly to the supplier and problems get fixed quicker.
The downside is the Telefonica charge far more than anyone else.
One other thing you can look at (if you are not a heavy user) is a flat rate 3G modem. Faster than most ADSL lines and mobile as well.