Updated to Android Studio 3.0 Please share missing items in comments.

A late answer but none of the answers here and here was right on the money for us...

So, here's our gitignore file:

#built application files
*.apk
*.ap_

# files for the dex VM
*.dex

# Java class files
*.class

# generated files
bin/
gen/

# Local configuration file (sdk path, etc)
local.properties

# Windows thumbnail db
Thumbs.db

# OSX files
.DS_Store

# Android Studio
*.iml
.idea
#.idea/workspace.xml - remove # and delete .idea if it better suit your needs.
.gradle
build/
.navigation
captures/
output.json 

#NDK
obj/
.externalNativeBuild

Since Android Studio 2.2 and up to 3.0, new projects are created with this gitignore file:

*.iml
.gradle
/local.properties
/.idea/workspace.xml
/.idea/libraries
.DS_Store
/build
/captures
.externalNativeBuild

Deprecated - for older project format, add this section to your gitignore file:

/*/out
/*/*/build
/*/*/production
*.iws
*.ipr
*~
*.swp

This file should be located in the project's root folder and not inside the project's module folder.

Edit Notes:

  1. Since version 0.3+ it seems you can commit and push *.iml and build.gradle files. If your project is based on Gradle: in the new open/import dialog, you should check the "use auto import" checkbox and mark the "use default gradle wrapper (recommended)" radio button. All paths are now relative as @George suggested.

  2. Updated answer according to @128KB attached source and @Skela suggestions