Twenty extensions are available now, ranging from overlays and follower leaderboards to those specifically built for games like League of Legends, Hearthstone, Overwatch and Destiny. Savvy followers might have seen their favorite content creators demo the new tools in the last few months (like Hearthstone streamer Trump in the pic at the top of this post).
Other extensions benefit creators directly: 'Gear on Amazon,' for example, earns Twitch Partners and Affiliates (aka streamers) commission through Amazon's Associates program when fans buy stuff promoted by their favorite content creator (see example below). This one is the first, but not last, extension to use monetizations: In a press release, Twitch noted that they will share more ways developers can make money through extensions at TwitchCon's Developer Day on October 19th.
Users can make their own extensions by heading to the updated Developer Portal, which will freely provide both asset hosting and fanout messaging to help promote their software. Developers can showcase their finished plugins in an Extensions Manager, which will act as a directory for creators to peruse. Twitch didn't announce when exactly all the extensions features will go live, but noted they will be available soon.