The best dictionary I've found is the Real Academia Española:
http://buscon.rae.es/draeI/ (also, it's free). Aside from that, I'm fond of
www.diccionarios.com, especially for translation resources (though, this one requires a subscription). However, I think that both are very good and I highly recommend them.
Sometimes I look at wordreference.com, but I prefer the systematic aspect of the dictionaries noted above.
Saludos,
Joseph