This site is pretty self explanatory with its name. Gif Porn Tube is a tube site, filled with animated GIFs - you know short little animations which are perfect for porn. Now I've never jacked off over porn gifs, but they are nice to collect or use as your avatar on porn forums.
The site is easy to use, with the most recent porn gifs shown straight on the homepage and the ability to change this to top rated or most viewed. Categories are fully visible on the homepage too and show you how many GIFs are in each one. Anal has 5361 files at the time of writing this!
The usual options are there for categories, such as brunettes and fetish with more niche specific options like fucking machines. Simply click on a category and you'll get what you are looking for. When you click on a GIF, you'll notice it loads in a new window, that's my only gripe.