Candy Arda
Arda as written by lintamande (moiety: candy), primarily around the time of the Silmarillion.
Flat Arda
Arda, original flavor. Most similar to the Tolkien text of all the candy Ardas.
Space Arda
The Silmarillion IN SPACE! Endore and Valinor are 25 light-years apart, with Valinor as a binary star system. Only Ainur are magic. Dwarves are unaging. Elves' souls are cortical stacks which develop in utero, and Elves can write software known as "blessings" to use with these souls. Osanwe operates via wireless networking between souls and cannot be used to communicate with non-Elves. Elven music is just good music. Elven marriage is performed by means of both parties downloading marriage blessings of their choice. Elven souls back up to Mandos, with the Silmarils intended as an alternate backup-catching method.
The tech level is higher, with Feanaro inventing the technology behind FTL travel (lightleapers, which go anywhere in 5 days), but not doing followup work. The war on Angband is fought using technologies including airplanes and later on nuclear weapons, eventually rendering the surface uninhabitable. Melkor forks and tortures uploads in rapidly accelerated environments.
Evil Arda
A Flat Arda in which Melkor focused more on messing with affairs in Valinor, such that the Elven crime rate was high enough to push Valar into regularly mind-controlling Elves, which screwed up the culture a lot. Most Elves in this setting are more terrible as people.