I wrote a Slack tool that implements a CFAR-style prediction market:

/predict create rain-2017-03-18 "it will rain tomorrow" tomorrow 40%
/predict rain-2017-03-18 70%
/predict rain-2017-03-18 80%
/predict resolve rain-2017-03-18 true

A contract is some proposition that isn't known now, but will be known later, at which point it can be resolved true or false. When you create a contract, you give some details about how you'll resolve it, a time when it will close and stop taking new predictions, and some "house odds". These odds try and start the predicting in a reasonable place, so no one gets a big windfall from noticing things first.

When the market closes, people earn points in proportion to how much they improved the group consensus. [1] This isn't a traditional prediction market in the sense of having buyers and sellers. The downside is that it offers windfall profits to the first person to react to news, but the upside is that anyone can make a prediction at any time without needing to have a counterparty.

The code is on github. I've been using heroku to host it, but it can run anywhere. If you're having trouble adding it to slack let me know.

[1] Specifically, you get (or lose) points in proportion to the log of the ratio of your prediction to the one before. So if you predict 60%, I predict 70%, and it's resolved true, then I earn log(70/60) points. If it's resolved false we recast these predictions as if they were predicting the opposite, so 30% and 40%, and I earn (lose) log(30/40) points.