The paid version is well worth the extra cost, IMHO. You get tons of cool icon variations and a lot of other neat features like layers and colors that aren't properly documented until you pay up and download a kit. For the price of one license, we were able to (legally) include access to all of their icons inside our product. A heck of a deal!

Having said that, you can self-host the free version without any restrictions (other than redistributing the font files).

Thank you, I missed their Kickstarter offer (just 20$) https://www.kickstarter.com/projects/232193852/font-awesome-...