A truly brilliant game available at the lowest price is ever been, straight from the publisher. You can choose either a Steam key or a DRM free version which is downloaded straight from their website.
The cheapest this game had ever been, the last lowest price was in 2013 £10.79. In the last several Steam sales it's always been 40% off which is £17.99.
In Kerbal Space Program you build rockets using parts and try to send adorable Kerbals in to orbit and eventually land on other planets.
It's my all time favourite game. I've owned the DRM free version since 2013 and bought the Steam version this year. Scott Manley's comprehensive video tutorial series covering your first rocket all the way to landing on the Mun.My Steam review