Libreswan is described as 'ibreswan is a free software implementation of the most widely supported and standarized VPN protocol based on ("IPsec") and the Internet Key Exchange ("IKE")' and is a VPN Service in the Security & Privacy category. There are three alternatives to Libreswan for Linux, Mac, Android and BSD. The best alternative is IPsec-Tools, which is both free and Open Source. Other great apps like Libreswan are strongSwan (Free, Open Source) and Openswan (Free, Open Source).
Openswan is an IPsec implementation for Linux. It has support for most of the extensions (RFC + IETF drafts) related to IPsec, including IKEv2, X.509 Digital Certificates, NAT Traversal, and many others.