EasyFX allows you to do post-production directly from the image editor, and it will automatically build your composition node tree. Once an image has been rendered you can easily start enabling effects, tweak values and manipulate the image to your liking.

Blendshare is a free image sharing add-on that allows you to upload renders, screenshots and GIFs directly to platforms such as Imgur, Dropbox, Google Drive and Twitter.

http://image.jpeg is a free Blender addon which allows you to easily open images from the internet by entering the image address/url. Super quick & easy to get e.g. reference images into your scene.