Once you get the hang of the query syntax and the user interface (which can be daunting at first), you can search through a large database of Spanish and Portuguese sentences to answer lots of different questions about these two languages, e.g. which preposition goes with insistir, which synonym of duro is most commonly used with trabajo, and many others.
Unlike Google, Mark’s corpuses allow you to search for all the grammatical forms of a word (just put the base form of the word in [brackets]), specify parts of speech (e.g. [v*] stands for any verb in any form), or search by proximity (e.g. find all adjectives within 5 words of ojos). They will also sort the results by frequency, which can be a real time-saver.