Diccionario RAE is a free program, which offers much more than just its name.
At first you might think it's a dictionary, and it is, but in the form of a search engine. You insert the word you are looking for and the program connects to the page of the RAE to offer you its results.
In addition to searching in the DRAE this program can search images in Flickr, for example, or normal and common searches in Google.
Perhaps it can be a cumbersome program since its interface has many buttons, but in return you have many tools available, all at a glance.