Shakespeare’s Words
An online dictionary of most of the words used in Shakespeare that don’t exist anymore or mean different things today, along with a list of every time they appear in his work. There is unfortunately a limit to the number of things you can look up for free but it resets every week, I think.
versemeter
Created by Keir Fabian, a very smart guy I follow on Quora, versemeter goes in-depth into the tricky technical workings of iambic pentameter. I don’t always agree with his analysis, but for the price of absolutely nothing you can learn about as much as you would if you paid $30 for a book on Shakespeare’s verse.
UVic’s Internet Shakespeare Editions
Contains the folio text of all of Shakespeare’s works in old and new spelling, as well as quartos of some of his works. Super useful if you need to compare printed editions of his plays but doesn’t have any glosses or annotations so it can be hard to understand (especially if you’re dumb like me and decide to read King John for the first time in the old spelling).
The Bodleian First Folio
Mostly cool because they’ve scanned every page so you get to see the actual book itself, and also features side-by-side transcripts of the facsimile with a modern-spelling version.
ElizabethanDrama.org
Read obscure plays that never get published in collections of Elizabethan drama! We can learn so much about Shakespeare’s time and work by reading his contemporaries’ works. ElizabethanDrama.org provides annotated versions of plays by a number of different playwrights that you probably won’t find elsewhere.
PlayShakespeare.com’s document library
I don’t care about most of the PlayShakespeare website, but their pdf copies of a bunch of seminal but out-of-print works on Shakespeare are a great resource.
The Henslowe-Alleyn Digitisation Project
This site is honestly pretty hard to navigate and the vast majority of the content hasn’t been transcribed, but if you’re willing to wade through kind of dodgy handwriting and Elizabethan spelling, they’ve got a lot of documents that are hard to find elsewhere.
The Internet Archive
An online library of thousands of out-of-print books. It’s free to join and you can check out books for 14 days. I have success finding books on there about half the time, but it’s a good alternative to physical libraries if your library doesn’t carry a lot of books on Early Modern drama.