To The Moon is a recent one that I like a lot, it's very different from most games, highly story-driven without a ton of actual gameplay. Certainly an interesting setup! Not to mention, the plot focuses heavily on autism so I think it's fitting for this forum!
http://freebirdgames.com/to_the_moon/
Cave Story is another good one that I'm sure most people have heard of, it's really difficult and frustrating at times but the exploration in it is so much fun!! It reminds me a lot of the old super metroid game. There's a ton of clones of the game also that are very fun as well.
Xenimus is one of the oldest and longest-running MMORPGs on the market, but it's relatively small and few people have heard of it. As far as I know it's basically developed and run completely by one person, so it does have its fair share of issues with bugs and hacking and stuff. The community was also pretty bad last I played it, lots of really rude/mean people. That said, it has some of the fastest-paced and most fun PVP combat I've ever seen in a MMORPG (on the small scale at least, larger fights are a bit hectic), and I've played a lot of them. The game world is really impressive too, there are literally thousands of "secrets" in the game to discover...the developer purposely added all sorts of hidden passages and things for players to discover on their own. It's definitely worth checking out if you like that genre!
www.xenimus.com