Cookie Clicker

Cookie Clicker is the idle game most people think of first. Orteil put it out in the browser in 2013, and it's still being updated. You click a big cookie, spend the cookies on helpers that bake more for you, and watch the count get absurd. The helpers start with grandmas, farms and factories and get stranger from there, all the way to portals. Upgrades push their output higher, and when progress slows you ascend, which resets the bakery in exchange for heavenly upgrades that last forever. The Steam version lists more than 600 upgrades and more than 500 achievements, plus mini-games and a dragon you can pet. The browser version is still free on the official site. The paid Steam release from Orteil and DashNet came out in 2021 and adds cloud saves, a soundtrack by C418, and community mods through Steam Workshop. If you've never played an idle game, start here. If you played years ago, a lot has been added since.