ImageOptim is a cool tool by Kornel Lesiński for Mac OS X used to optimize images. Why would I want to do that you ask? It reduces the size of images so they take up less disk space. This means if you are working on a website, it will load a little faster for your users. If you are working on games, it can reduce the amount of data being sent to the GPU which can speed up the loading of textures [make sure you are using lossless compression of course].
Photoshop is especially bad at creating small files when it saves them. It’s incredible the number of images on the web that can be optimized to save everyone time and money. Anyone running a website should consider using ImageOptim or a tool like it on their images.