Rated "Very Positive" from 240 reviews, I found this to be a simple, unassuming, but likeable little indie retro game- a 2D shooter/platformer with exploration and a sci-fi theme. The play mechanics are sound, the graphics are endearingly cute, the music is really nice, and the plot pulls you along for its short 6 hour-ish playing length.
What's more, I was surprised to find you get a free Steam Key for "Polarity" if you enter your email address when the game launches. This is a Portal-style 3D puzzle platformer, currently £6.99 at Steam. It's rated "Very Positive" from 4,147 reviews, though recent reviews are "Mixed".