The Good Judgment Project is the OG competitive prediction website, yeah? Metaculus was released in 2015 and Manifold, Kalshi, and Polymarket were all part the crypto boom. I suppose there's Long Bets (2002?) but that always had a barrier to entry

Good Judgment Project apparently started in 2011 as part of an IARPA tournament. I first learned about through The Economist, which frequently mentions the crowd consensus on forecasts related to Economist articles.