Here’s another useful resource for students of Spanish.
The UltraLingua Online Dictionary will let you look up English translations of Spanish words of course, but it also has a neat useful feature for helping you read entire webpages in Spanish. The site provides a “dictionary enabling” feature, where you provide a web address and the page loads with every word of the text as a clickable link which pops up the translation. This way, you can read as usual, but if you see an unfamiliar word, you can click on it for an immediate translation.
Existing hyperlinks are undisturbed, while the rest of the page gets transformed as described. My only complaint so far is that it appears to mangle non-English characters, such as letters with an accent over them in Spanish.
According to their terms, usage is free for light, casual use, but free use has a limit on the number of lookups per day, after which a subscription would be required.