Isn't the Yoigo one 25 Euros for 3GB? The benefit is that this is on their own network or on Movistar which has good coverage. (all at 3Mbps)
Yoigo - Internet para llevar. Accede a internet en un ordenador desde donde quieras.
Simyo (as I mentioned in the other post) is now 18 EUR for 1GB or 25 for 5GB (3.6Mbps)
simyo - Habla gratis entre número simyo, sólo 8cent/min al resto.Siempre.
Blau is 21,50 for "unlimited" though they throttle after 1GB. (3.6Mbps until then)
blau - Qué es.
Masmovil is getting decent reviews and is a good choice for occasional use at a low 1.5c/512KB. (3.6 Mbps) They have an English website, and English speaking customer service. They are also supposed to have some tariff of 18 Euros per month for 1 GB.
MSmovil: The smart mobile choice 
Apart from Yoigo, all of the above are on Orange, who don't have the best coverage and are getting a bit congested in places. Some people are now preferring the move to Vodafone who can get you HSUPA speeds. I think the best one for that is Pepephone who will give you 300MB for 7 Euros or 1GB for 17.99.
pepephone.com - El operador móvil que vuela - Contratación
Just watch out for minimum consumption on all of these, in case you want to keep your SIM active for the next trip. Simyo is still good es there is still no consumo minimo, even though their customer service is shocking!
Also see:
Comparativa tarifas internet móvil desde ordenador