It appears to me that StrongSwan and LibreSwan are the two main viable products now-a-days. strongswan vs openswan has one good comprehensive comment with some comparisons between StrongSwan and LibreSwan. StrongSwan seems to win the argument in that link.
But to be fair, I saw Paul Wouters, who represents the LibreSwan project at RedHat, talking today at the Security session of LinuxCon in Toronto. He put up strong arguments for opportunistic encryption and continuing on with the original project's by line of 'encrypting the internet'. Paul's site is https://nohats.ca/.
But there is overlap between the two because 'ip xfrm' forms the basis for kernel tools of ike/ipsec. So if you need extra certificate stuff, then libreswan or strongswan are needed. But some encryption stuff can be performed with neither present.
Libreswan is a fork of Openswan, searching for "strongSwan vs. OpenSwan"
should give you a broad range of impressions and meanings.
Both strongSwan and Libreswan have its origins in the FreeS/WAN project.
Open/Libreswan are still much closer to its origin, where strongSwan
these days is basically a complete reimplementation.
The current strongSwan architecture was designed initially for IKEv2
almost 10 years ago, but since 5.x is used for IKEv1 as well. It comes
with an extensible, well scaling multi-threading design, and has its
focus on IKEv2 and strong authentication.