Not sure if there are any decent free places to advertise, but then free and good don't usually come in the same sentence.
For holiday rentals, I recommend
www.holiday-rentals.co.uk, I've known a few people that have had good success getting holiday rentals from there, although its not free.
Long term rentals you may want to try placing a text advert in the Costa Blanca News? I think its 25c per word or something along those lines. Another option is placing a postcard advert in your local convenience stores, quick save and the like - that works.
Best of luck
