I'm having a blast (literally) playing Besiege. It's a sand-boxy physics simulator; the premise of the game is that you're building siege engines to besiege map objectives. At present, there's no limitation on parts. Only one chapter and a free sandbox mode have been implemented so far - the game is in Early Access alpha. My war machines have universally been awkward hodge-podge contraptions that routinely tear themselves apart when I try to fire weapons, it's great. At present, it's $7.79 (CDN), and I'd say it's certainly worth that much. I made a video, too!
I had enough funds leftover from selling off all my trading cards, so I decided to get this for free. I don't have a lot of free time to learn something new at the moment, but I have enjoyed the first few levels. I've also made incredibly unorthodox and highly-unlikely-to-ever-really-work machines. So that alone was worth the price haha