The best place to look for crossovers with Buffy is 'Twisting the Hellmouth' (http://www.tthfanfic.org/). You'll see the link to the Supernatural fic on the front page, then you can narrow your choice down by filtering on your favourite characters.

'When Worlds Collide' (https://www.fanfiction.net/s/9018850/1/) is an excellent crossover with NCIS. When a kidnapped Tony DiNozzo is rescued by legendary - and supposedly dead - serial killers Dean and Sam Winchester, the NCIS team takes over the Winchester investigation. What they find is a file filled with contradictions and impossibilities.

An interesting series of crossovers is 'Supply and Demand (http://tari-roo.livejournal.com/11584.html)' by tari_roo It's an AU, in a world not too dissimilar from the SPN one we know, except that Kinetics and Empaths exist, and Dean is an Empath. There are three completed stories, which follow on from each other:
Unwanted (http://tari-roo.livejournal.com/7002.html#cutid1), SPN/NCIS Gibbs didn’t want or need an Empath. But Director Vance was insisting and his team were avoiding him, more than usual, so when the T&E Centre called to say that Dean Winchester was available, it was time to pick up the unwanted Empath.
Repeat Business (http://tari-roo.livejournal.com/12110.html) SPN/Criminal Minds Mitch Koravi was dead, to begin with. But when a man is killed using his exact MO, the BAU have to eat humble pie and bring back the Empath told them their case was not closed, Dean Winchester
Stolen (http://tari-roo.livejournal.com/16636.html) SPN/Stargate Atlantis When an impossibly locked door is keeping the Trust from treasures unknown, they arrange to steal an Empath so that their Kinetics can ‘crack the safe’. Unfortunately for Dean, he’s the unlucky Empath and the safe is in Pegasus.
A fourth fic was started, continuing Dean's adventures in the Pegasus galaxy, but that hasn't progressed very far.

More SPN/NCIS
NCIS and the Haunted Computer (http://tassosss.livejournal.com/50239.html#cutid1) The Computer Did it.
Mostly NCIS with guest appearences from Sam and Dean, but there are a couple of sequels which are linked from the first one.

Happy reading!

Wow thanks for all of these will definitely check these out 'When Worlds Collide' sounds really good.

Thank You

SPN/TWD Xover
Haven by miss_grey
http://archiveofourown.org/works/2953769/chapters/6529115

Clean by Jenasys Loveless Lovecraft
https://m.fanfiction.net/s/8742826/1/Clean

Both are Destiel and the second one is also Caryl.

SPN/NCIS Xover
Way To Kill by readersdigest
https://m.fanfiction.net/s/4281158/1/Way-To-Kill

No pairing for this one and its preseries.

Edited 2015-08-13 09:50 pm (UTC)

Thanks for these. 'Haven' sounds like a good read.

My favourite SPN/Dark Angel crossover is "Of Desire and The Status Quo" by grumkinsnark , it's very long but it's an epic and captivated me completely. Dean meeting Alec is so well done and it's a brilliant crossover.

https://www.fanfiction.net/s/5382298/1/Of-Desire-and-the-Status-Quo

Thanks and yep defiantly long nearly 200,000 words, hopefully keep me busy for a couple of hours and I do like my Alec and Dean fics :).

http://otp-destiel.livejournal.com/571.html

Wow that's a lot of chapters, thanks will definitely check it out, sounds like a good read.