Biggest question: why would anyone want to play a remake of Pokémon Crystal on the Game Boy Advance when HeartGold and SoulSilver already exist on the DS? A lot of reasons, it would seem.
Pokémon Liquid Crystal, a massive hack of Pokémon FireRed, has been in development since 2007, around when Diamond and Pearl first landed. You might have heard of it over the years, but if you haven't, it almost completely clears all the data from the FireRed ROM and loads the space with tons of new goodies that rival even the DS entries. There are now three regions to explore, 386 Pokémon to capture, 20 gym badges to earn, recreated sprites from scratch, all new music, and a bunch of story twists compared to the original Crystal.
On the game's development page, you can grab the current build, Beta 3.1, but you'll need either a flash cart or a GBA emulator to play, obviously. I salute such a monumental task, and I hope Poké-fans can get a lot out of such a passionate, fan-driven endeavor.