A Big Bang is a style of challenge in which authors write fics of a set minimum wordcount, and an anonymous claims process allows artists to select a story they would like to illustrate. Both fic and art will then be posted together, often one pair per day during the challenge's posting period.
Supernatural's first Big Bang was the SPN_J2_Big_Bang, which started in 2007 and has had dozens, if not hundreds, of entries every year since. The challenge has always been multiship, despite how modern fans tend to read it as a ship name: in 2007, SPN RPF consisted almost entirely of J2, so SPN-J2 was shorthand for SPN/SPNRPF.
The original Big Bang challenge in Harry Potter fandom had a minimum wordcount of 50,000 words, but most subsequent challenges have lowered this significantly. The SPN-J2-Big-Bang (on LJ) has a minimum wordcount of 20k words. So does the DeanCasBigBang or DCBB, the other largest bang, which started in 2010. Some Big Bangs have minimums of 15k or 10k; Mini Bangs are often in the 5k - 10k range. The Supernatural Reverse Bang also started in 2010, and reverses the order of things: artists draw an art prompt which authors claim and write a story based on. The minimum wordcount for this started out at 5k and dropped to 3k; artists may choose or not to create additional art for the story. In many modern bangs it's common to have "two-track" challenges: the challenge runs with both mini and big categories, and artists may be expected to produce more art if they claim a story from the 'big' track. Supernatural fandom even invented the Mega Bang: very high wordcount tracks of 50k, 75k, and 100k.
The SPN-J2-Big-Bang, the DCBB, the SPN Reversebang, and many smaller challenges started out on LiveJournal. As fandom has shifted in large part over to Tumblr, individual challenges have shifted over entirely, partially, or not at all. Many new challenges have been started on Tumblr. Lots of challenges maintain both an LJ and a tumblr and post announcements, or at least links to announcements, on both. Some challenges also maintain a Dreamwidth, Twitter, and/or Discord server; the DCBB even has its own wiki.
Big Bang style challenges have sprung up centered around specific characters, ships, groups, or genres within Supernatural fandom. Many but not all of their works can be found on AO3, often in challenge-specific collections. Tumblr-run challenges often encourage or require participants to post to AO3; older LJ challenges from when AO3 was new and not as widely used often had most entries posted directly to the participants' LJs and linked from the community.
Check out this exhaustive spreadsheet of Supernatural Bang-style challenges, past and present; it excludes only challenges which folded without any entries completed and challenges which did not try to pair up all participants (these can be found on the next tab, which is a not-exhaustive account of other SPN challenges).
For news of Big Bangs in all fandoms check BigBang Index on Livejournal, BigBangIndex on Dreamwidth (includes many LJ communities as well), and The Big Bang Blog Project on Tumblr. Here's another list of Supernatural bangs and other challenges on Tumblr (and LJ too), as of 2016. You can also check Big Bang tags on AO3, though not everyone tags.
Big, mini, or reverse bangs which have been ongoing for at least three years as of 2018:
- SPN_J2_Big_Bang started in 2007, for stories featuring any Supernatural character or Jared or Jensen pairings, or gen; story minimum of 20,000 words
- Dean/Castiel Big Bang started in 2010 and features stories with Dean/Castiel as the central pairing, or Dean and Castiel as the central characters in a Gen fic.
- SPN Reverse Big Bang started in 2010 and reversed the usual process. In this challenge artists produced work, for which writers then produced stories of at least 5,000 words.
- Gabriel Big Bang started in 2010/11. Story minimum length 15,000.
- Wincest Big Bang ran in 2013, rebooted in 2018. 20k word minimum. Main pairing must be a Winchester - does include Adam.
- Sastiel Big Bang started in 2013, for stories with Sam/Castiel or Jared/Misha. Now alternating a mini/big bang with a reversebang format.
- Sammy Big Bang - started 2014. Both Big Bang and Mini Bang format for Sam-centric stories; 5k or 15k.
- Sabriel Big Bang started in 2014. Two tracks: minimums of 10k or 20k.
- Team Free Will Big Bang Started in 2014. Sam, Dean, and Cas; 5k or 15k.
- Meg/Cas Big Bang Started 2014, minimums 5k or 10k.
- Asexual Supernatural Mini Bang Started in 2014, minimum 5k.
- Wincest Reverse Bang Started in 2015, minimum story length of 4k.
- Wincest Big Bang Started 2015, minimums 5k or 10k.
- Dean Winchester Big Bang Started 2016, minimums 5k or 10k.
- J2 Reversebang Started in 2016, minimum 4k.
- Dean/Cas (previously Destiel) Reverse Bang Started in 2016, minimum 3k.
- Samifer Big Bang Started 2016, mimimums 5k or 10k.
- Crowley Big Bang ran 2011-2013, story minimum length 15,000. An entirely new Crowley Big Bang started on Tumblr in 2017 and in 2018/19 is a reversebang.
Some early bang challenges, dormant as of 2018:
- SNcross Bigbang ran from 2008 to 2013 as a challenge for Supernatural fics which crossover with another text. Stories are greater than 20,000 words.
- Gen Big Bang ran 2011 - 2014. Story minimum length 15k, later mini track of 5k
- SPN Slash Big Bang ran 2010 - 2013. Stories must have a central slash or femmeslash pairing. Story minimum length 15,000
- Sassy Minibang - a fic challenge for >10K words fics with accompanying art centering around Sam and Castiel (aka Sassy)or Jared and Misha.
- John Winchester Big Bang
- Adam Milligan Big Bang
- SuperWho Big Bang - for SuperWho fic - Supernatural/Doctor Who crossover.
- J2-Kane Big Bang - minimum 10,000 words. Stories must feature Jared, Jensen, Chris Kane or one of the characters they played.
- Before or After Big Bang. Minimum 5,000 word challenge featuring stories set before or after the main Supernatural canon.
- SPN Hardcore Big Bang - Stories from moderately kinky to extreme hardcore. Mini bang of 5K, Big Bang 15 k.
- SPN Femmeslash Big Bang Minimum 10,000 word fic either canon or RPF featuring female characters in slash relationships.
See also Fanfic challenges & exchanges