Front Facing Flash
The built-in camera on your iPhone or Android is one of the best Selfie apps because it is instantly available. The trick is to use the front facing flash to get better lighting on your photos.
The iPhone 6s and newer includes a front facing flash option that will slash the screen at a very bright level, but one that matches the picture's lighting, to take a better selfie in low light. In the iPhone camera app, switch to the front facing camera and tap on the flash icon.
Most Android phones including the new Galaxy S8 include a front facing flash option as well, which will flash the screen to light up your selfie. The options vary from phone to phone, but on the Galaxy S8, you just need to tap the flash icon.
Free, you just need to use your camera.