Plugins for Minecraft 1.8.2: 127 items in the Plugins section. Compatibility is checked against the files, not the description: only items with a build for exactly these conditions stay in the list.
Plugins handle almost everything behind the scenes on a server: ranks and permissions, land protection, economy and shops, anti-cheat, map rotation, menus and voting. You install them by copying the .jar file into the plugins folder and restarting the server; most create their own config on first launch, which is what you then edit. The card lists supported cores and game versions, the author and license, and all releases are gathered on one page — you can update the plugin right from there.