For iPhone
Measure Master Pro Calc ($14.99)
Feet-Inch-Fraction and Metric Conversion Calculator.
Scala ($2.99)
All your scales in one app allows you to measure printed drawings when you don’t have your physical one.
Converter: Units & Currencies (Free)
Convert units easily in an intuitive user interface.
Swatches (Free)
View your Swatches as RGB or Hex values and even see the nearest paint and Pantone colors!
Protractor Edge – Angle Measurer (Free)
A simple protractor. Just put your device on the angle you want to measure. Then drag the two lines to align both sides of the pitch.
Angle Pro (Free)
Measures angle with +-0.2º to +-0.3º accuracy depending on your device with this handy angle meter and level app.
For Mac
Units Master ($11.99)
A universal unit conversion calculator allows you to instantly calculate and convert US and metric units of length, distance, and area.
Pixelmator ($29.99)
Full-featured and powerful image editing app for the Mac. An economical alternative to Photoshop’s offerings.
PureRef (donation encouraged)
Drag and drop images from anywhere into PureRef and have all your reference images in one place. Set PureRef to always stay on top of your workspace and resize to your liking.
Uberlayer (open source)
With the Uberlayer app, you can put floating images on top of your computer screen. Great for cross-referencing.