Rogue-likes: the randomized frontier.
FTL's randomness makes for some incredibly entertaining gameplay as you attempt to make the most of your trip from one side of the galaxy to the other to save the Federation, all while being chased by looming, innumerable rebel forces.
You will extermish pirates to varying degrees of glee and bitterness. You will guide that lost civilian to their destination, find out it was a rebel trap, and smile. You will relish every victory.
You will have to sell that Burst Laser III so you can upgrade your engines. You will open that Mantis escape pod because you are desperate for someone who can board ships. You will lose the last bit of your ship's HP to an unfortunately timed missle. Failing to make it to the end of a roughly 6-hour playthrough is what makes your victories significant.
FTL is visually very simple, a hallmark of its indie production, but as much as it could benefit from a bigger-than-1280x800 resolution or perhaps even 3D graphics, I love how I can run it on my integrated graphics laptop.
FTL is a tremendous value and important landmark in the history of crowdfunded games. Were I to wake up in an alternate universe where I had yet to create a Steam account, I would buy FTL again, for around three incarnations, anyway.