Votum for Slack - Crowdsourced lists with upvotes, inside Slack
Frozen Goodies - An advent calendar to get the chance to win daily prizes
Must-Read for Slack - Build controllable must-read lists for your team in Slack.
25DaysOf - A customizable advent calendar, for everyone
Corpus - Curate, take notes and share your reading list
25 Days of Vim - A Vim Advent Calendar