Your local library is probably the best source of free books full stop. With the rise of digital technology, many libraries have started loaning out ebooks through systems like Overdrive. Check out your local library and see if they have a system to lend out ebooks.
2. Websites and Databases Dedicated to Out-of-Copyright Books
There are a wide range of websites and databases that compile books that have gone out of copyright, and that you can read legally for free. Probably the most well-known example is Project Gutenberg, but there are plenty of others.
3. Kindle Books
Not only are there Kindle editions of out of copyright works, but many authors offer their books for free for various reasons. Maybe it's a temporary promotion, maybe they make the first book in a series free to hook new readers, or maybe they are trying to get on a bestseller list. Searching the free ebooks section of Kindle and finding a gem can be tricky, but worth it.
4. Social Media
If an author or website puts an ebook up for free for a limited time, it can be difficult to catch the deal. One solution to this is to search hashtags like #freeebooks on your favorite social media sites. Most social media sites are arranged in chronological order, so you will be kept up to date with the latest deals.
Any cool tips for legally getting free ebooks? Leave them in the comments.