libtins is a high-level, multiplatform C++ network packet sniffing and crafting library.
Its main purpose is to provide the C++ developer an easy, efficient, platform and endianness-independent way to create tools which need to send, receive and manipulate network packets.
It uses a BSD-2 license and it's hosted at github.
High level != inefficient
libtins was designed keeping efficiency in mind at all times. In fact, it is one of the fastest packet sniffing and interpretation libraries available. The benchmark section contains some actual measurements of how fast it works.