The Internet Archive: Seriously one of the most useful websites out there. Has both the Wayback Machine and a huge, totally free library of media, including literally millions of books available to download or borrow. Wikipedia: Seems pretty obvious, but gives you the world's knowledge at your fingertips, all for free.
TV Tropes: Waste an afternoon and ruin your life. The Anarchist Library: Collection of anarchist works and thought, all available for free and simple download. spooky-ghostwriter's Stand Stats Maker: Make your own stand stats! Wide-band WebSDR: Listen to radio transmissions online. I recommend starting with 4625 kHz. Petalica PaintsChainer: Really interesting site that uses AI to color your line art. Good for interesting watercolor style. The Charley Project: Database of missing persons cases from the early 1900s to today. RationalWiki: Online wiki on pseudoscience and right wing extremism. Library Genesis: Huge online database of books, papers, magazines, comics, and other texts. If this URL doesn't work, try one of their alternates. Video Toolbox: Useful and fast online video editing site that's especially good for quick cuts and conversions. Friendship Ended Generator: Make your own "Friendship ended with" pic! The Cutting Room Floor: Online wiki of all the weird finds and beta editions in video games. Pac-Man Google Doodle: Fun fact, within a week of this doodle people wasted almost 5 million hours playing this at work. The Super Simple Userbox Maker: Make simple userboxes. Available both for wikis that use Wikipedia-style markups and in HTML- it's what I used on my about me page. reddit: Interesting forum-like site made up of millions of different sub-communities.