OPC Servers
OPC (OLE for Process Control) — Standards and specifications for industrial communications in Runtime.
OPC was born in 1996. It is a series of standards and specifications for industrial communications in Runtime. Over the years, several specifications have been released: OPC DA v.1, OPC DA v.2, OPC DA v.3, OPC XML-DA and OPC UA.
KEPServer
Kepware is the owner of the KEPServer OPC Server, featuring drivers to communicate with most PLCs — see their catalog.
RSLinx
RSLinx is Rockwell Automation's OPC Server. Functionality depends on the version: Lite, Professional, OEM or Gateway.
OPC Factory Server (OFS)
OFS is the current OPC Server for Schneider (Telemecanique) PLCs.
DAServer — Intouch / Wonderware
Intouch FSGateway, successor to the former OPCLinx.