You have to sign for at least one year with most of the ISPs. Wanadoo and Jazztel are among the ones that don't have that condition. Note that Wanadoo has the worst reputation in Spain and I wouldn't go for it.

As for the special offers ISPs usually have, if you cancel the contract before 12 months they will probably ask you to return what you got for free (gifts, special discounted rates for the first months...).