Elysium / Nostalrius
The oldest Vanilla World of Warcraft private server currently online, Pre-TBC honor system intact, Blizzlike world and dungeons scripts, fully working quests and game events. Warden anticheat and crude language filter replicated from retail. They also offer a Vanilla server database on their site to help players.
Made up of the old Nostalrius Vanilla WoW private server and Valkyrie WoW and offering multiple realms that cover both PVP and PVE.