One of the things I missed when I switched from a DSLR to iPhone photography was the ability to capture long exposures. The iPhone camera app simply won’t let me do it. I tried a few camera apps and sticked with Slow Shutter Cam that helped me to get around this limitation.
With Slow Shutter Cam you can take long exposures. And I mean long! Up to 30 seconds and even longer if you switch to bulb mode. Best of all, you can even edit the motion blur strength after you took the shot.
In the November 2016 update, the developers even added manual ISO control. So you can turn down ISO, which in turn lengthens the exposure time, and get low noise shots even in low light situations.