The Man With No Name
By: Frostfyre7
Doctor Who/Firefly crossover
He hadn't really counted on getting into a pub brawl over the color of his coat on some speck of a planet, or signing up with a crew of petty criminals. Still, he'd always wondered what really happened to those Lost Colonists from Earth...

End Of the Line
By: shewhoguards
Harry Potter/Discworld crossover
Hell was, Snape decided, a crowded railway platform. Post Deathly Hallows. Contains spoilers.

but it seems you've beat me to the punch. Here's my contribution:

Sol Umbrae by Dream Bolt
When Twilight Sparkle and her friends are sent to go to the strange world of Mobius to defeat an ancient evil, they end up encountering a strange, spiky blue creature named Sonic. The ponies easily make friends with Sonic, but soon find that not all of the people on Mobius are friendly. Mysterious events lead to misunderstandings, while an evil villain known as Dr. Eggman is setting his eyes on Equestria as his new target.

And amidst all of this lies a great mystery, a mystery that will shed some shocking revelations on how Equestria and Mobius share a great deal of history, on the origins of the being called Solaris, and on a dark threat that both worlds know all too well.

A well-constructed Sonic/MLP crossover. Though it has its roots in Sonic '06 (gasp!), it does a good bit of worldbuilding, tying the aforementioned Sonic '06 and a lot of the main canon sources into a cohesive timeline--as well as seamlessly merging MLP (this was written after the Season 3 finale) into the universe. SPaG is not perfect, but it doesn't detract much from the story. I'd highly recommend it.

Yay, goodfic thread! I have a handful of lovely slash fics (or just fics that include slash) to offer you.

First up, from the Free! fandom, Blue Lotus, White Lotus by shionsheart. Set in the desert AU as seen in the S1 ending, this is primarily a Rei/Nagisa fic, but Makoharu also features (as does Rinsuke). I should warn you that it does include themes of slavery, forced prostitution, and war, so if you're sensitive to those, you may want to check out the next thing on my list instead...

Namely, <a href="http://archiveofourown.org/chapters/2094253?showcomments=true">Nothing Gold Can Stay by perkynurples. I'll just drop the summary here and let it speak for itself:
"Bilbo Baggins led a rather peaceful life, thank you very much, until an old acquaintance decided to turn it upside down, and he found himself agreeing to take a job that’s… let’s say not exactly up his alley, and might eventually cost him a little more than his treasured cozy lifestyle. Who would have thought tutoring a slightly menacing monarch’s more than slightly overbearing nephew could prove to be such an… adventure?"

If you want something action-based, I have The Hetalian Job by RainyMeadows (along with its sequels). This is more in the "awesome fic that includes slash" category; Death the Kid finds Italy lost and scared in the Black Forest and things spiral out of control from there. Content warnings for graphic violence (including some torture), swearing, blood, and in one case suicidal thoughts.

Last, a femslash fic: Girl She Loved by Fabulolz. Literally every other fic in the Free! fandom I've looked at/into has been either some variety of slash or a het pairing, so this jumped out at me automatically. Furthermore, it's about Gou's two friends Chigusa and Hoshiko, who don't get lots of screentime for obvious reasons. (Content warning for emotional abuse.)

Here's one: The Spirit of Redemption by Myetel. It's for Mass Effect. (Link is SFW, but the fic is not. There are plenty of explicit romance scenes, but they're decently done, can be skipped, and are clearly marked with scene breaks/author's notes so you can easily skip past those.)

I don't want to scare you away from this and its prequel, but there are several caveats to this fic. The first is that it's a Mass Effect AU, so much so that the canons are side characters to the OCs.

Drawback 2: There are more than a few changes to canon, so biotics become psychic wizards even more than in the games.

Drawback 3: Plotholes and Deus Ex Machinas. There are a lot of 'em, especially toward the end of the fic (it kind of jumps the shark for its final arc). Worst offender is a two-year time skip where a romantic pairing breaks up with no foreshadowing or explanation, and a spoileriffic thing involving Prothean nanobots.

Drawback 4: A bunch of cultural stuff is entirely made up for the fic, up to and including snippets of language. It's based on the influences the canon drew from, so I consider this a strength, but others may feel differently.

All of that said, The Spirit of Redemption is still amazing. I don't read a lot of fanfic, but this has great characterization and nearly flawless SPaG across several million(!) words. Some of it is less-than-great, but there's enough good stuff that it's more than worth the read.