A couple Fire Red came to mine when I saw this thread:
1) Pokemon Light Platinum - 2 continents and 2 pokemon leagues to explore, graphics look like the first DS Pokemon titles in a way, and plenty of Pokemon, secrets, and rivals to find.
2) Pokemon AshGray - From what I've seen it follows the Anime Show, not exactly sure how far along it goes (I'd imagine at least the Kanto Seasons). Lots of neat graphical edits as well.
3) Pokemon Liquid Crystal - A remake of Pokemon Crystal on Steroids. From the latest version I played a while back, all of the base game is complete (as well as some extra stuff) and the author's even making a 3rd area to explore after you finish Kanto.
But that's just a few of the excellent hacks for Fire Red.