WARNING: This plugin is still in a very early state of development, many bugs exist.
Lifeless is an open source SMP implementation of single player's Hardcore mode. It is designed to integrate with Minecraft's game modes as closely as possible so that the end user or administrator won't be able to tell the difference between hardcore mode and a built in gamemode. You can access the source code here, pull requests are appreciated.
Lifeless's version of hardcore keeps track of every change a player makes in hardcore mode; then, when they die, it does its best to undo them. Version 0.2 works, though neither integrates completely nor provides configuration options.
For Server Administrators: Simply download the plugin and put in the plugins folder. Everything should run be done automatically. Eventually you will be able to set hardcore as the default game-mode via server.properties though right now players have to be put in it manually. This plugin shouldn't interfere with other plugins, since it doesn't register any commands. It is confirmed to work with one player though multiple player testing has been minimal.
For Users: To enter hardcore mode, simply use the vanilla command /gamemode <user> <gamemode> with a gamemode of 2. Unfortunately, while this requires access to the /gamemode command, so a moderator will probably have to put you in hardcore mode.