The following tools are available in either mobile format for Android phones (via the Google Play Store) or as a Windows desktop application:

Unit Conversion Utility to convert between many common unit types
Pipe Dimensions Utility to calculate pipe ID, flow area and other properties for standard nominal bore piping.
Pump Sizing Utility to calculate pump properties.
Horizontal Vessel Sizing Utility to calculate wetted surface area and liquid volume of a partially filled horizontal vessel.
Vertical Vessel Sizing Utility to calculate wetted surface area and liquid volume of a partially filled vertical vessel.
Linear Interpolation Simple utility to perform a linear interpolation between a set of numbers.