Arguably the most well-known indie game of all time, Cave Story+ features a completely original storyline wrapped with personality, mystery and hours of fast-paced fun. Cave Story+ is an action-adventure game that is critically acclaimed indie game. Overflowing with unmatched charm and character, Cave Story+ takes you into a rare world where a curious race of innocent rabbit-like creatures, called Mimigas, run free. Run, jump, shoot, fly and explore your way through a massive action-adventure reminiscent of classic 8 and 16-bit games. Cave Story+ features higher resolutions, tons of new art and music, along with content not available in the original version.
Great story, adorable characters, great gameplay, great music, great art, lots of choices to make, multiple endings, innovative weapon leveling mechanic, cross-platform, free, just an overall excellent game.
I haven't played it for a while, but I can't remember anything negative to say about it, except maybe that it gets pretty hard at the end, especially if you go to "hell".
Of all the GNU/Linux games that have been coming out lately, I almost regret that this one is free, since it's easily one of the most worth paying for!