What's a good long-exposure app for the iPhone? I'm looking for an app that lets me take long exposures of 10 seconds or more. This can either be one long exposure, or multiple shorter exposures, like 0.5 seconds, combined together by the app.

This is on the iPhone 5s running iOS 8. Not jailbroken, and don't intend to.

The app needs to have a timer to avoid camera shake caused by pressing the shutter button.
If you're on iOS 8, the new camera API made possible by Apple allows true manual exposure control over the pictures taken. I would suggest Manual - Custom exposure camera. It's a really good replacement for the native camera app as well.