OTW, the ones behind Ao3 frequently gives out Ao3 codes as prizes. To know when this happens, follow them at the following links.

OTW's main news website: http://transformativeworks.org/news
News comm: http://otw-news.livejournal.com and http://otw-news.dreamwidth.org
OTW Tumblr: http://transformativeworks.tumblr.com/ (Runs monthly promotions frequently with AO3 invite code giveaways)

Once again, please delete your post after you get your code, edit it with a strikeout or tag it yourself (non ops can tag posts) to say you got your code. Thank You