I just read an amazing fic, a crossover of Merlin and Supernatural by ariadnes_string . I would have never thought that a crossover between these two shows worked so well, but it did.
Title: Two Boys from Kansas in King Uther’s Court, 1-7
Rating: pg-13, gen, no pairings.
Fandoms: Supernatural/ BBC Merlin
Characters: Sam, Dean, Merlin, Arthur and various canon BBC Merlin characters.
Word Count: ~36K, all part
Warnings/ Spoilers: Set Merlin mid-S1, no spoilers for S2; set mid-S1 for SPN, but very vague spoilers for S2; also, a version of a SPN canon character from S3-4 shows up eventually, with some very vague references to SPN 3.09; and there's a non-spoilery, blink-and-you-miss it shout out to SPN 4.18.
I won't give anything away, just read it... it's really, really worth it, I promise. :o)