Cave Story. Definitely. No computer should lack its glorious installation. Along with Dwarf Fortress.
** In addition, a fun Roguelike / Strategy RPG is Gearhead 1 &/|| 2. They both feature random plot generation, lots of giant mecha to use, perfectly balanced on-foot fighting (that doesn't feel like an add-on) and they're both totally non-linear. Well, the main plot is fairly non-linear, but you're in a world and can go wherever you wish. There's also alternatives to permadeath and the ability to turn off auto-save (and thus, permadeath.)
Gearhead 1 is older and complete. Gearhead 2 is newer, fancier, but buggier and less complete. Both are extremely fun, and come in Tiled Isometric and ASCII flavors.
http://www.gearheadrpg.com/A few good memories; modding my Gyro Mace (for on foot) to both be on fire and shoot lasers. A weapon called the "Atomic War Club" that detonates an atomic explosion on impact. Modding my giant Savin mecha to have six wings and be capable of extraordinarily fast flight speed. Taming an army of fire monkeys. Creating a sentient robot that's twice as good at me at everything and has her own mecha. Making a non-sentient robot out of only glue.
** I know you already know about Elona.
** Synaesthete is a DigiPen student game of exceptional quality. Essentially a beat/rhythm game with a twist- you move and explore interesting, linear landscapes while fighting off a variety of strange, nightmarish things by using your tracker. It's simple but extremely fun and the music is top-notch.
https://typo3.digipen.edu/index.php?id=986Also present is very nice poetry between most rooms, that presents a sort of disjointed story or concept according to the level you're in. A fusion of music and magic.
** Lastly, Haxima in the Nazghul engine is a retro computer RPG in the spirit of the Ultima series, particularly Ultima V. Don't let the low version number fool you- it's a long adventure, with an ending, and rock solid all the way through. The design of the world, areas, and enemies put professional game developers to shame. It's probably one of the most well-polished and balanced freeware game I've played, up there with Cave Story.
http://myweb.cableone.net/gmcnutt/nazghul.htmlThe puzzles are clever and witty. The enemies and dungeons are numerous and captivating. Your allies are actually fairly interesting, and your character is very changeable. Despite its quality it receives apparently little attention. Though, if you didn't like older CRPGs such as Ultima, you're probably not going to enjoy this, as it feels older than it is.