Here is the 2014 Big Giant List of free (or low cost) game development resources.

Fonts

An often under looked aspect of games is picking a font that fits the aesthetics of the game.

Da Font A mega library of mostly free fonts! Be careful to check the licensing of any font you end up using from this site.

Graphics

CG Textures An awesome resource containing textures for everything from bricks, grass, paper, and plastic to blood spatters, animals, and fire.

Glitch the Game Glitch is a game that was released in 2010 and closed down in 2012. The art resources and code from the game have been released to the public domain under a Creative Commons CC0 1.0 Universal License! Interestingly enough, the team behind Glitch went on to create the very popular Slack tool. You will find great art and music assets on this site.

Open Game Art - You can find tons of free pixel and game art on this website. You can find a complete set of sprites for a 2d game on OpenGameArt called Platformer Art Deluxe that is worth checking out.

Lost Garden You have to check out these beautiful Zelda like 2d terrain sprites offered for free by Daniel Cook. You can definitely tell that the sprites are made by a professional and are just awesome to look at.

Icons

Making app icons is a giant huge pain in the butt. Here are some resources that make it less painful.

App Icon Template Download a free photoshop template that will create all the icon sizes for your IOS game. This is a great timesaver!

Fix My Assets This website will generate Windows Phone, Windows, and IOS assets from a single uploaded SVG or PNG. I haven't tried this site yet but it does look promising.

Source Control

BitBucket - BitBucket offers completely FREE unlimited Git repositories for up to 5 users. In my experience, BitBucket offers the same experience as GitHub but is FREE! For non open source projects I highly recommend BitBucket.

UI

Mobile Game Gui - Graphic Burger offers a free template for a mobile game GUI that is of professional quality. The free GUI is themed for a more light hearted casual game, so it doesn't work for every type of game.