PhotoPen is a very useful app for field use. It allows you to markup a photo with boxes, clouds, lines and text. You can describe a safety issue, generate an RFI, or simply add a time stamp.
PhotoPen has a simple concept: mark up your photos and share them on your iPhone, for free. You can choose pictures from your library, or take snapshots with the camera. The photos can then be annotated freehand, or using a resizable cloud, square, or circle shape. There is also a calendar-shaped watermark button, which allows you to date and tag your photos with custom resizable text.