Openswan is an OpenSource implementation of IPSec for the Linux operating system. Is it a code fork of the FreeSwan project, started by a few of the developers who were growing frustrated with the politics surrounding FreeS/WAN. (Openswan has no subsequent capital letters and no slash).
With the ending of the FreeSwan project (their deciding that OpportunisticEncryption wasn't happening in a hurry), OpenSwan is the defacto FreeS/WAN replacement. There is also another fork called StrongSwan, focusing on PKI and certificates/policies; see the StrongSwan page for a comparison chart between the two.