You read Viticci on how the MacStories team uses Slack to keep everyone on the loop and Seth Clifford shared how he uses Slack to receive important notifications and do other neat tricks. I've been using Slack as a notification hub for a while now and here's my tight collection of integrations.
After you create your Slack room, click on its name in the top-left corner, look for Configure Integrations. There you'll find services natively supported by Slack such as Pingdom, which I use to monitor if this site is down or anything unexpected happens, and Twitter, which you should connect just to expand tweet links.
The real integrations arrive when you connect Slack as an output for IFTTT recipes and Zapier tasks. With the former you can get weather notifications, NASA's image of the day or learn about new Twitter followers. With the latter, Zapier, I'm particularly fond of integrating it with Mention, a service that tracks keywords across the web—with this zap, and RescueTime, a service that measures how productive you are based on the things you do in your computer, here.
For more information on how Zapier meshes with Slack, check this post by Marissa Daily where she covers a myriad of integrations between the services.
I also learned about a service that matches Mention, built around Slack, called Notify. It has been great so far and I suppose that they'll monetize on specific sources in the future, but so far it is free. I learned about Notify in a Product Hunt list where you can find other tools.
I'm intrigued with Slack bots and how you can develop bots for quick actions, such as looking up a word in the dictionary, turning your one-person notification panel into a chat room where you interact with bots that perform tasks for you. Perhaps that's a glimpse into the future.