Hercules/390: the New Users Documentation Effort (Site) - A volunteer effort by many authors to create a Documentation for the Hercules emulator, coordinated by Cory Hamasaki, divided into Primary, Non-Install and Advanced topics.
N.U.D.E. (Site) - User documentation for Hercules.
Your Private Mainframe (Site) - Introduction and links about the Intel-based Hercules mainframe emulator, as well as an MVT tutorial and MVS 3.8 binaries and add-ons.