Big Bang
A Big Bang is a style of challenge in which writers 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, as 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 writers 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.
These three 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 more challenges have started entirely 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. Alyndra has compiled an 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 still not exhaustive).
