Liquid Crystal is a hack of Pokémon FireRed Version. It's a lot of fun from what I've played of it so far. It's pretty much an upgraded version of the original Crystal, with some extra characters and areas added.
You get to choose between Gold or Kris to play as, which is pretty nice. Both the sprites look really good, and have completed back-sprites for when you're in battle, too.
The interiors have been reworked a little, and look good.
The VS Seeker was reworked into a PokéGear, which is pretty cool.
Since the time and date are shown at the bottom now, all the functions have been shifted up by one and the last functions is now used as a Game Boy Player. It switches the music and sounds between the updated GBA ones and the ones from the original Crystal Version.
The environments are pretty great-looking; just like an updated Crystal should look like. There's also a day-of-the-week and day and night feature.
The interiors of the buildings are nice-looking, too.
The graphics in-battle are pretty nice, too; some of the sprites have been reworked to look more like their older counterparts.
I mostly wanted to go over early-game stuff to avoid potential spoilers about updates from the original game. It's a lot of fun to play, but more fun to see all the little upgrades and differences for yourself.
If you're looking for something a little different yet familiar, I'd suggest giving this one a try. The little updates to the game make it really enjoyable, and it's one of my favorite Pokémon hacks to play.