ReadLang.com is a freemium website with a number of tools to aid in the reading of foreign language texts. You can import any web-based text or book into ReadLang and read it in the ReadLang interface, where the translation of each word or phrase can be revealed with a single click. The English translation for the word you don’t know is displayed inline with the text. As you go about clicking words you don’t know, these words get added to a queue which you can see when you reveal the tab on the right side of the page. You can hide and reveal the dictionary in this panel, so that you can spot check the automatic translation.
Translations are driven by Google Translate, so there will be mistakes. That’s one really perilous aspect of this website. You don’t want to learn cards verbatim from Google Translate EVER, because you don’t want to learn something wrong. It’s best to either verify your translations in the dictionary first and edit the card so that it’s accurate, or if you can’t verify it in the dictionary because it’s a phrase or has several meanings, you can get your translations blessed by a native speaker (you could ask someone or post it as an exchange request to LingQ.com).
Once you are finished with the text for now (it saves your place so you can return to where you left off), you are ready to start dealing with the unknown words and phrases. You can export the new words to get a file that can be imported into Anki, or you can use the ReadLang website itself to study the words. The ReadLang flashcard system is fairly nice. It tests both Spanish->English and English->Spanish. It shows you the surrounding context of sentences where the word was saved using a cloze display. And if you click the Edit pencil on the back of the flashcard, you can see the dictionary page for the word.
ReadLang flashcards use the spaced repetition system, with a twist. One drawback of other SRS software is that if you miss a few days, your review cards will pile up and you’ll be faced with a mountain of cards to get through. With ReadLang, you can choose how many cards to review per day. See more about this system here. Also, ReadLang uses word frequency information to automatically prioritize which cards you learn first. Very nice!
For sources of Spanish text to import, check out the post Where can I find Spanish books and text to read?