Come for the excellent Suomi Warders series about a small merc unit on a mission in 3052 and the fact every 'Mech fight was gamed before it was written. Stay for the Warders 3057 and some of the most insane BT fiction you will ever read: Urban Legends.
Erratic_Cheese's work is, quite simply, the most hilariously screwed-up fanfiction you will ever read for this or any other universe. Palerider's Urbanmech Brigade, the PUB, is here to do their merc-y thing. It'll all end in tears of laughter and metaphors your mind will reject in horror. Though it might be best to skip Preacher Man.
Synopsis: Drop everything, there's magical lostech to be found! Seven regiments worth of mixed mercenaries in armor, infantry, and 'Mechs bound for the Periphery. Then we add a Galaxy of Clan Jade Falcon, a couple of Goliath Scorpions, the lost heir to the Taurian Concordat, and it all gets pretty ugly. But do remember, the Terran Hegemony kept many secrets, even from itself. And there's always a way things can be worse.
Synopsis: An undercover operation spirals horribly out of control when the not only does the target prove to be a lot bigger then expected, but the operative decides to take matters into their own hands. The end result is something akin to a sandbox crime game (hence the title) but (as expected) with Giant Robots. The first part is now offical BattleTech fiction, with the rest being a Spin-Off (after a fashion).
Synopsis: The Reunification War-era Taurian Concordat battleship TCS Samantha Calderon misjumps centuries into the future. The knowledge and personnel she has on board may change the Concordat's future... if the rest of the Inner Sphere doesn't destroy her first.
Synopsis: Stefan Amaris has taken Terra, his coup of the Star League underway. But a single special battleship, SLS Praetorian, with human intelligence and the ability to subvert other drone craft, may be able to save the Inner Sphere from the coming darkness just yet.
The Misadventures of Darius Davion, parts 1, 2, 3, and 4 by Tokage
Synopsis: A Miles Gloriosus trading on his Davion family name basically ends up being the cause of no end of trouble, occasionally encountering famous personages from the official storyline proper and the occasional in-universe Historical In-Joke. A conniving Dirty Coward and Butt Monkeywho gets what he deserves for his arrogance, and yet perhaps the luckiest sod in the Inner Sphere at the same time, Darius can basically be described as a Composite Character of every one of the eponymous Blackadders at the controls of a Humongous Mecha. If that description doesn't sell it, turn away now. Has a few spelling and grammatical errors and could do with a bit more formatting, but it doesn't really detract from the highly entertaining storyline.
Synopsis: In 3020, a chain of events lead the Federated Suns and the Draconis Combine a front row seat as the backwater Wretched Hive of Antallos is invaded by an unknown force utilizing technology not seen since the Star League... or ever. The new force, the Coalition of Sovereign Nations led by a man named Jack Ryan, is extremely secretive about their origins, but is willing to radically change the Inner Sphere to ensure their safety.
Synopsis: Mass Effect crossover. Aleksandr Kerensky has led the war weary Star League Defense Force on a two year long journey into exile. A chance decision leads to a long awaited discovery, and final proof that humanity is not alone in the cosmos.
Comments: There are several interesting changes to the canon that make this interesting, even for a Fusion Fic/AU, one of the most notable being Humanity making a peaceful first contact with the Quarians, as opposed to the usual canon and violent first contact with the Turians. All together a good read.