Not an app but a website, and one that seems to be fulfilling a need for a great diagram program whereby you can create a range of diagrams from pre-drawn elements.
- Basic Shapes
- UML 1.0 (Unified Modelling Language)
- Entity Relationships
- UI Form Components (User Interface)
- Upload your own images (PNG, JPG & GIF)
You do need an account (free) but you could easily screen grab and crop for small diagrams