###Mac OSX Development/Admin Tools
0 - XCode
XCode, or at least the XCode command line extensions, are at the core of any development work that you are going to do on a Mac. You'll need to install it to get things like the C Complier and bootstrap the build environment. If you are going to be doing iOS or Mac OSX development, get the entire package from the App Store. If you are not, just install Homebrew, and it will prompt you to install the XCode CLI tools.
1 - Homebrew
OSX Unix Package Manager. The site is at brew.sh. You can use this to install things like Git, Ruby, Python, etc.