Pokemon Pyrite is a Pokemon Crystal hack. After being so successful in selling millions of the first Pokemon games of Generation I like Red/Blue/Green/Yellow, Nintendo continued to create the next generation with Pokemon Crystal.
Pokemon Ultimate End is a good Pokemon Crystal Hack. It is not for those who like nice graphics because it uses Pokemon Crystal Codebase with GBC graphics.
Pokemon Pikachu Edition is a remake of Pokemon Crystal on GBC that is used Pokemon Crystal Codebase. English Version is available.
Pokemon Prism is an awesome Pokemon Hacks for those who like playing Pokemon on Gameboy Color platform and based on Pokemon Gold.