Metroidvania. It's a sub-genre of gaming made famous by, well, Metroid and Castlevania, games renowned for their 2D exploration and backtracking. It's made a bit of a comeback in recent years, with games like Shadow Complex, but it usually doesn't look this pretty.
This is indie game Celestial Mechanica, available now for $5. Made by Roger Hicks (rComplex) and Paul Veer (Super Crate Box), it's got two things I like in my video games: great music and blue skies.
It's on both PC and Mac, and you can grab it at the link below.
Celestial Mechanica [Official site, via IndieGames]