I have several suggestions for funny stories, however they are all short. But, you did say you would take any lengths, so here are my favorite funny stories.
Date Night. Dean/Cas Summary: It's date night, and not hell or high water is gonna ruin that. http://deancastiel.livejournal.com/1670233.html#cutid1
Supernatural Romance. This is Gen actually, and one of the funniest things I have ever read. Summary: Announcing the release of the twenty-fifth book in the Supernatural Romance series by Carver Edlund. http://cupidsbow.livejournal.com/418915.html
Prince Castiel and the Quest for 100 Orgasms. Dean/Cas Summary: Castiel knows about how gladiators can win their freedom by winning 100 fights, so Castiel has decided to operate under the assumption that if Dean has 100 orgasms, he might let Castiel go. http://luchia13.livejournal.com/274949.html
Start Quoting Shakespeare and We're Done. Dean/Cas with a side of Sam/Gabriel. A/U Summary: In which Dean has the hots for a librarian named Cas, Cas may or may not have the hots for a mechanic named Dean, and Gabriel joins Sam in the peanut gallery in the hopes that he might just get to do a horizontal tango of his own. http://archiveofourown.org/works/244582
Put on Your Dancing Shoes Dean/Cas. Summary: Dean needs a favor, Castiel is the ideal fake boyfriend, and the Winchester/Campbell clan... helps. http://archiveofourown.org/works/180892
Accidents Will Happen This is Gen and the main character is an outsider. Summary: Some Goth teenagers accidentally resurrect John Winchester. No, seriously. http://archiveofourown.org/works/35067
As for crossovers, I know of a really good one with the Marvel Universe. It is a two story arc called: Dean Winchester, Agent of SHIELD. Level 6. Summary: Dean is beginning to think that a stint as Tony Stark’s bodyguard is just a rite of initiation. This is Gen as far as Dean is concerned, however it is Tony/Bruce. http://archiveofourown.org/works/1092559 Things We Lost in the Fire. Summary: Over the years, Dean thinks he may have lost just as much as he’s gained. He means to change all that. This has a bit of Dean/Natasha, but it doesn't do into details. This is not a funny story. http://archiveofourown.org/works/1359166
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Date Night. Dean/Cas Summary: It's date night, and not hell or high water is gonna ruin that. http://deancastiel.livejournal.com/1670233.html#cutid1
Supernatural Romance. This is Gen actually, and one of the funniest things I have ever read. Summary: Announcing the release of the twenty-fifth book in the Supernatural Romance series by Carver Edlund. http://cupidsbow.livejournal.com/418915.html
Prince Castiel and the Quest for 100 Orgasms. Dean/Cas Summary: Castiel knows about how gladiators can win their freedom by winning 100 fights, so Castiel has decided to operate under the assumption that if Dean has 100 orgasms, he might let Castiel go. http://luchia13.livejournal.com/274949.html
Start Quoting Shakespeare and We're Done. Dean/Cas with a side of Sam/Gabriel. A/U Summary: In which Dean has the hots for a librarian named Cas, Cas may or may not have the hots for a mechanic named Dean, and Gabriel joins Sam in the peanut gallery in the hopes that he might just get to do a horizontal tango of his own. http://archiveofourown.org/works/244582
Put on Your Dancing Shoes Dean/Cas. Summary: Dean needs a favor, Castiel is the ideal fake boyfriend, and the Winchester/Campbell clan... helps.
http://archiveofourown.org/works/180892
Accidents Will Happen This is Gen and the main character is an outsider. Summary: Some Goth teenagers accidentally resurrect John Winchester. No, seriously. http://archiveofourown.org/works/35067
As for crossovers, I know of a really good one with the Marvel Universe. It is a two story arc called: Dean Winchester, Agent of SHIELD.
Level 6. Summary: Dean is beginning to think that a stint as Tony Stark’s bodyguard is just a rite of initiation. This is Gen as far as Dean is concerned, however it is Tony/Bruce. http://archiveofourown.org/works/1092559
Things We Lost in the Fire. Summary: Over the years, Dean thinks he may have lost just as much as he’s gained. He means to change all that. This has a bit of Dean/Natasha, but it doesn't do into details. This is not a funny story.
http://archiveofourown.org/works/1359166