The concept of this thread is simple, really. Know any good fanfiction lately? Post a brief description of the fanfic (if it focuses on a specific pairing, which is almost always the case, also include it), URL (as the title), and of course the name of it.
Here's some to get you started (plus an example on how to write your posts). it doesn't have to be this long:
AVATAR: Last of The Black Core. by Dearing
Summary: Essentially a reimaging of ATLA with an alternate history setting (the Fire nation is a Nazi Germany renamed The German Unified Nation, or G.U.N, and controls 2/3 of the world, The Earth Kingdom now The United States and Canada, or U.S.A.C, and The Water Tribe reimagined as The New Soviet Union) set in a alternate Second World War that lasts 100 hundred years, rewritten characters (you won't believe how masterfully the author rewrites the characters while keeping their core personalities intact), and Gundam-inspired mecha taking the place of benders (don't worry; it's not as cheesy as it sounds)!!
The writer really did a good job fleshing out the back story and making the Avatar universe his own staying true to the series's roots (For example, Sozin's comet is replaced by a gigantic orbital laser aptly called the COMET system that virtually acts as the same plot device as in the series). It also adds some interesting Original characters, like the supposedly blind and enigmatic Major Artirius Meridiano.
But the centerpiece of the story are The Rangers; massive, 15-metre tall Mobile Suit knockoffs that serve as the analog to benders in Last of The Black Core and are heavily inspired from every conceivable mech genre, from Zuko's Evangelion-infused Dominance ranger to Zhao's Big O Colossus Ranger. Even Aang get's a Gundam; the Armored Vehicular All-Terrain Assault Ranger, or AVATAR; which, of course, serves as the centerpiece of the entire story. Hell, Even Appa is upgraded from a giant bison to a colossal all-terrain transport that flies called the AVAT (short for Armored Vehicular All-terrain Transport) The mecha battles themselves are spectactular, especially in the story's version of The Siege of The North season finale. There's also a sequel out, which covers the second season of ATLA (though it hasn't been updated since June)
In short, if you love Gundam, Avatar, or both, check out Dearing's AVATAR: Last of The Black Core!