Double-click on a UML element to the right
to add it to the diagram
Advanced users: select "Custom Elements > New..."
to create entirely new element types
You can directly mail diagrams from UMLet with Ctrl+M (Mac: Cmd+M)
The mail can include gif and pdf versions as well
Zoom with Ctrl+MouseWheel; Lasso with Ctrl+Mouse (Mac: Cmd instead of Ctrl)