cdnjs is a free distributed JS library delivery service.
ConvertKit is an email marketing tool built for content creators.
Open Graph is a protocol that is used to integrate any web page into the social graph.
OpenResty is a web platform based on nginx which can run Lua scripts using its LuaJIT engine.
PayPal is an online payments system that supports online money transfers and serves as an electronic alternative to traditional paper methods like checks and money orders.