You can find my intro to the Corpus of Contemporary American English here, but there’s a related site called http://www.wordandphrase.info that will let you enter a bunch of text and then tell you all about them.
Here’s what it does:
* It highlights medium and low frequency words (and create lists of these words you can use offline)
* You can see how “academic” the text is
* You can click on a word and get its frequency, frequency-per-genre (spoken/fiction/magazine/newspaper/academic), its top collocates (nearby words), synonyms, and related words.
* At the phrase level, you can highlight a phrase and it’ll show you related phrases from COCA. The example Mark Davies gives is clicking on “potent argument” would show you “strong/persuasive/convincing argument”, which are all more common.