Engineer's Handbook Contains useful conversion factors, resources and data associated with engineering and physics. Free on iTunes store.
RealCalc - A Realistic Scientific Calculator for Android
Engineering Professional - Combines all of the formulas from Chemical, Civil, Electrical, Environmental, Hydrology and Mechanical Engineering formulator apps. Over 650 formulas allow for determining everything from calculations on shafts to calculation on loads on beams.
FingerCAD - the first CAD application for technical drawing with fingers assisted by computer. With "FingerCAD" you can draw houses, bridges, floor plans, mechanical components, geometrical figures, anything that can be designed with a common PC CAD.
iEngineer - contains a complete specifications for over 300 U.S. and metric bolts and screws.
Below are provided image resources freely available to be used for personal or educational purposes (image citation(s) may still need to be provided).