Libwebsockets covers a lot of interesting features for people making embedded servers or clients

• http(s) serving and client operation
• ws(s) serving and client operation
• http(s) apis for file transfer and upload
• http POST form handling (including multipart)
• cookie-based sessions
• account management (including registration, email verification, lost pw etc)
• strong ssl PFS support (A+ on SSLlabs test)

You can browse by api category here

A collection of READMEs for build, coding, lwsws etc are here