Android App Store (not iOS) has the PhET app for free, usually $0.99 [google.com]
Run by UC-Boulder, PhET is a website that has bunches of simulations for science and mathematics. The app contains all of the HTML5 sims, making them available offline, plus is updated as new HTML5 sims are developed. I'm a high school science teacher and use them regularly.
HEADS UP: If you've downloaded the 1.0 app, they've abandoned it. Upgrade to this ASAP while it's free.
YMMV territory: after I put the app on my phone, I got a popup from google play giving me a $3 off $3 coupon, which I used to purchase Stardew Valley
I have no idea if this is replicable, but thought I'd share (and brag about my deal on Stardew Valley -- thanks powerfuldoppler).
EDIT: after seeing posts, it's unlikely that this coupon was triggered by the app "purchase," as only some posters are receiving coupons and nobody's received the same as I. App's still useful, though, especially if you have students (or are one).