Foursquare's Swarm check-in app has just updated to add widget support for iOS 8 users. The update squashes some bugs and lets the app work faster.
Checking in from the Today section in Notification Center is fairly straightforward. Swipe down the Notification Center, find the Swarm widget, and tap to check in. The widget will also show you friends that are nearby.
You can download Swarm 1.3 right now from the App Store.