I've found libpcap to be a very simple API to use given the power of what it's able to do. However, if you're trying to work with things like TCP streams, writing the reordering logic can be a huge timesink.

The only non GPL solutions I've found are libntoh[1] and libtins[2]. Both are nice, but libtins is much more powerful out of the box, taking care of the pcap code as well. There's one or two things I don't like with the API of libtins, but overall it's a pretty nice library if you want to capture / inject packets.

[1] https://github.com/sch3m4/libntoh [2] http://libtins.github.io/