I've used several different companies from Murcia airport including Centauro, Europcar and others. They are all good for service and value - you really can't go wrong when hiring in Spain (far better than the UK).
I have usually used one of the agents like www.carjet.co.uk and searched on google for the best deals. If you want to check out Centauro directly their website is www.centauro.net
Hope this helps
Stu (www.esendionline.com)