The author is extremely talented, with a beautiful style and an excellent grasp of the characters and their relationships (and her Technobabble is great too). Writes largely on the relationship between Agatha and Zeetha and the OT 3 of Agatha/Gil/Tarvek, but also about the first generation characters and other relationships; there's something for every GG fan. Varied ratings, from G to R.
Seconded. Excellent grasp of the relationships and dynamics involved. Can and will make you cry. Can also be found at 
Synopsis: A Crossover Fanfic between Girl Genius and Stargate SG-1, which asks the question of what happens if Agatha Heterodyne were to be trapped in an alternate universe and SG-1 discovers her before she learns her heritage. Hilarity Ensues.
Synopsis: Utter crack AU, Vanamonde/Tarvek. Based loosely on the book of the same title in Gil's library, with Tarvek and Vanamonde taking the places of the dastardly Sheik and innocent Heroine respectively. Lots of implied nudity.
Synopsis: Girl Genius historical fiction. Maxim becomes a Jäger; the Storm King and the Heterodynes maneuver against each other on and off the battlefield; and Otilia is thrown into both cultural and internal conflicts. The author is particularly good at recreating Girl Genius-style dark humor and paints the old Heterodynes convincingly as Sparks, terrors to Europe and beloved in their own town.
Characters: Maxim (central); Otilia; Euphrosynia, Bludtharst, Gradok, and Clemethius Heterodyne; Ogglespoon, Ognian, miscellaneous original characters
Synopsis: Klaus Wulfenbach's parents were, as the Secret Blueprints tell us, Sparks who ran their holdings well and used their gifts to benefit their people... and had three sons, but the other two haven't been seen in years. Klaus is a construct. This is the story of Katherina Wulfenbach, and how it happened. (Somewhat medical. Not too graphic.)
Characters: Katherina Wulfenbach; the rest of her family
Synopsis: Fusion crossover AU with the Marvel Cinematic Universe. Agatha's life went in a different direction, Tony Stark is her great-grandson, the Super-Soldier Serum is actually the Jägerbrau, and Maxim is meeting all the Avengers. The background of both canons requires some tweaking, but it really works stunningly well.
Tags: Steve/Tony (just a crush so far, but it's only three chapters in)
Synopsis: A three way crossover between Girl Genius ,DC Comics and Buffy the Vampire Slayer with some elements from other sources (though its classed as a Buffy/ Justice league fanfiction), Where Xander, in a typical Halloween World/YAHF plot dawns the costume of a gentlemen adventurer and ends up the Son of Agetha and Gil, who is quickly swept away by an annoying cigar smoking imp into the DC universe. His life becomes swept up in a whirlwind of science, sex and superpowers as he goes from 'The Zeppo' to the leader of a mighty empire.
Synopsis: An alternate universe where instead of going into hiding, Barry heads to Mechanicsburg when he hears Klaus is there at the start of the Wulfenbach takeover. Klaus talks Barry into helping him conquer Europa for its own good ("You're talking me into things?"), and young Agatha, Gil, and Tarvek strike up a friendship on the still-in-construction Castle Wulfenbach.
Tags: Kid Fic, AU, Politics and Intrigue, Canon/OC Romance
Recommended by: PK (in support of a previously unsigned rec)
Synopsis: Girl Genius meets Steampunk James Bond (as the titlesuggests.) Featuring Ardsley Wooster's adventures before the comic begins. The other lead is an original character whom the author unabashedly writes as both very human and precisely as awesome as she ought to be. Lots of action and plotting, lots of fun for the readers.
Synopsis: Sequel to "On Her Undying Majesty's Secret Service". The author sums it up quite well: "A mysterious nobleman, a high-stakes game, nobles from across Europa invited to a mysterious meeting, and a Wulfenbach crashes the party..." Just as good as the first, but fair caution — it's not abandoned (I checked!), but it is a long-term WIP and the author is rather busy.