Here be crossovers. Genres and fandoms are noted after the title/author.
Tony Stark meeting Martha during the Year That Never Was: it's a terrific idea, and is perfectly executed. It's true to both sources in equal amounts, and all the characterizations are dead-on (especially the Tony-voice). Martha needs a gun, and Tony wants to help, all his other efforts leading to just more death and destruction. It's a harsh and cruel story, but also just the right touch of uplifting, and very plausible and very good.
Hysterical beyond words, this is both a pitch-perfect little slice of episodic Black Books goodness, and a seamless integration of Jack into that world. Manny has an eBay addiction, Jack wants that 100% genuine alien tech and no flirting is too far to get it, Fran and Manny are enamoured, and Bernard hates everyone, ever, in all universes – so nothing new, there. The voices are dead-on, there are ridiculously funny twists, and you just can't pass up a fic that gets Bernard this right: Bernard hated the internet, in more than just the generalised way he hated everyone and everything on Earth. He didn’t mind the pornography aspects, he said. It was the idea that it could bring people across the globe together to share interests and swap views in a vast network of common humanity. That was downright offensive. An absolute must-read.
A note-perfect crossover. Everyone involved is written spot-on, and it's absolutely hilarious to boot. Romana shows up at Hogwarts as the Defence Against the Dark Arts teacher: Hermione is sceptical, Snape is put-out, and Harry and Ron find out something about Romana that leaves them in a bit of a tizzy (and in particular, I love that Harry and Ron are written so true to self, somewhat clueless but never getting bashed). There's vampire hunting, school saving, and lines of sheer brilliance for all involved, such as In Potions, Harry scribbled, alien!!!!! in his textbook and passed it to Hermione under the desk.. This is just marvellously done.
Boys (or, Down With the Patriarchy!) by uktechgirl. (Doctor Who/Angel crossover, femslash, Ace/Fred, PG, ~3,400 words)
This is an absolutely brilliant story. The crossover really and truly works (and I'm not generally a fan of Angel, so it had to work extra hard), bringing in Ace, who has something of a Slayer about her, and intertwining Fred and Ace's storylines, as well as the mythologies of both shows, all without trying too hard and forcing things that don't fit. All of the voices, from both teams, are dead-on perfect (Angel at the beginning is particularly hilarious), and the style is intriguing and manages to be appropriately serious while also having a light touch. And Ace and Fred make a fabulous team, with the relationship aspect played softly, hinting at something that could be amazing.
A wonderful merging of the two universes, this uses the conceit of time travel as known in the Potter world and the resolution of The Parting of the Ways to create a cracking little story. We get a really strong characterization of post-war Hermione, including lots of great details about her life and her job to set the stage, and a Rose who saves the day, again, and with style. This was written for me in a ficathon, but managed to do so much more than I thought possible with the set-up, and in particular I love the way it's subtly inspired by the quote I requested: The weird sisters, hand in hand / Posters of the sea and land / Thus do go about, about. A very satisfying story.
This story works so incredibly well. It gets Harriet Jones just right, and the whole piece is written perfectly, in style and details. It's short but powerful, and very satisfying, leaving you with a Harriet trying to find her place (and a woman) in a world where War is always one step ahead, and calling the shots. The universes are blended seamlessly, and really gets at what can be so powerful about crossovers, merging the whole idea of the two worlds together, and the resulting consequences. As a bonus, there's a tiny piece with Death and Rose attached to the end of the fic, also beautifully written.