I didn't notice until this morning, when I saw that 165,000 people were playing it, but hidden object game The Tiny Bang Story (opens in new tab) is completely free on Steam today. The game is from 2011, and challenges you (gently) to click around its lovely and imaginative environments to find hidden objects and solve puzzles. I've played a bit and it's a soothing and serene way to spend your time.
It'll only be free for a couple more hours—you can grab at until 10 AM Pacific Time today (Tuesday). So click it quick!