Maintenance is probably the most customizable free maintenance plugin for your Minecraft server you can find. It runs on Bukkit/Spigot, BungeeCord, Sponge as well as Velocity.

Its features include:

A custom motd as well as server icon, that will be shown during maintenance
Start- and endtimers, which will enable/disable maintenance mode after the time is up
A '%TIMER%' variable usable in the pingmessage, to show the time until a running endtimer finishes (other variables and tricks are explained in the configuration file's comments)
A maintenance whitelist, to grant specific players the ability to join while you're working on your server
Nearly all messages are editable via the language file, given in a multitude of different languages
Features specifically for Bungee/Velocity
- Only want to enable maintenance on a single server? You can also do so by using the /maintenance <on/off> <servername> command
- Link multiple proxy instances through a MySQL database connection, so you don't have to change maintenance on each proxy by hand
- Use PlaceholderAPI placeholders with the MaintenanceAddon (only for Bungee)

A full list of commands, permissions and configuration options can be found in the wiki listed below.

Wiki
Downloads (GitHub)
Development builds (out of order at the moment)
Changelogs
Issue tracker/bug reports
Discord

Other forum threads

Compiling

To create a working jar yourself, simply clone the project and compile it with maven (by using mvn clean package in your IDE console).

Licence

When modifying the project's code, do that on a public repository while also keeping it under the same license (see GNU General Public License).