For now, just add/fill in sources you would like to document. As more information gets posted, rmb177 will organize them into logical groups rather than having one long list.
PICOUNT provides a current tweet count for all of the four main tweet markets: readldonaldtrump, whitehouse, potus, and vp. Along with the current count, the site provides the average tweet count per 24-hour period and uses that to compute a simplistic prediction of the final count (or pace). Users who use this pace metric for predictive purposes may often get wrecked. This site was founded by user Abe Simpson
Predicting Politics is a blog authored by jipkin. Besides providing commentary on current markets, jipkin provides two guides that have great information on the tweet and polling markets. Many newcomers to [PredictIt[ are referred to these guides when asking questions about either market type.