DevTools is an application for iOS developers.
Simple calculations that you could look up on the internet, but don't want to bother.

Convert colors between RGB, Hex and CMYK and UIColor

List of all fonts on the device, and metrics about the font at a particular size.

Convert numbers between Hex, Decimal, Octal, Binary and Scientific notation.

ASCII Table

SHA1 and MD5 a string.

List of Emoji with Hex ID.

String format specifiers for Objective C.

Block syntax