Here are some examples from this year:
O2 are adding a Halloween element to their 'Be More Cat' campaign
& using today's promoted trend on Twitter to get people to enter a competition by searching #HalloWin to find a secret code (it's 666)
Carling
Crest
Honda
Oreo
Tide (part of a series - see more by searching for the #ScaredStainless hashtag - which are also being used on Digital Out Of Home sites)
Lowes
Marks & Spencer
Pizza Hut
More Vine examples at Unruly and The Realtime Report
Audi on Instagram
Gatorade on Instagram
Domino's are asking people to make a 'Box o Lantern'
This spooky Scarecrow will protect your pizza from scavengers. Make your own #BoxOLantern and share a pic. pic.twitter.com/G7EuQrJ7k1
— Domino's Pizza (@dominos) October 29, 2013
See patterns to cut around on their dedicated Pinterest page
Asos are doing a 'Tweet a Scary Selfie' competition on Twitter
🎃👻🎃👻 Happy Halloween! We want your scary selfies. Send them over using #TodayIs and you could win a £100 voucher. 🎃👻🎃👻Innocent
— ASOS (@ASOS) October 31, 2013
Fruit bat pic.twitter.com/B8DNStsGZ8
— innocent drinks (@innocentdrinks) October 31, 2013
Any good ones I've missed? Tell me in the comments or on Twitter (I'm @dancall)
See last year's examples here