This game called Sky Rogue is coming to Early Access this week. I've been watching it since one of the earliest alphas started on the developer's website and it was still free (it became a $10 buy-in in January). The game: Ace Combat with circa 1994 flat-shaded SEGA Model 1 graphics. I've watched the game develop into a decently filled-out experience with a lot of substance behind its style.
So far however I have only purchased one Early Access game -- Interstellar Marines.
I think this game has been in development in one form or another since like 2005 or something. It even suffered a failed Kickstarer, but from what I can tell the developers are still trying, offering healthy updates every few weeks. Think Rainbow Six 3, or maybe any tactical FPS with open level design from between 1997 and 2005, but sci-fi and space and stuff. The second "campaign" level they currently have playable feels like a mix between a Rainbow mission and System Shock 2. Mostly IM is a multiplayer affair right now. The developer pushes its updates through multiplayer first, then co-op, then finally into the few parts that are playable solo. I also think they just worked the whole game into Unity 5.