Every developer has their favorite set of tools. I have written about a few of them so I'll link to those tips. Here's what I use mainly for iOS app development (in alphabetical order):

AppCode

AppCode is worth using just for the refactoring tools, quick-fix functionality and inspections.

Charles

A proxy server that lets you examine HTTP(S) requests made by (other) apps.

Dash

A third-party documentation viewer. Viewing documentation in Xcode used to be very slow. Dash was much faster, but the speed difference is not so significant anymore.

ImageOptim

A very handy tool for reducing the size of your PNG files.

Kaleidoscope

A pretty diff tool

MacVim

I write most of my code in MacVim. Still one of the faster way to enter and edit text.

Paw

A tool that lets you make HTTP(S) API requests and display the results in a nice format. Supports extension which generate code you can copy and paste to make the same requests in your project.

Regex

I built this app so I can work out regular expressions as well as test them.

ShrinkIt

Similar to ImageOptim, but for PDFs.

Sketch

An excellent vector drawing app. Has great support for generating images at different resolutions — especially useful for app icons and in-app assets — with the click of a button.

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