This mod allows you to use your mobile device as a "Second Screen" or a "Companion app", your device can show informations from the Minecraft instance. Currently there is a native Android (4.0+) app and a universal webapp which can be used on any device with a modern internet browser, but the universal app is currently lacking a few features.
The mod is designed to be installed on the server only, but you can also install it on your Minecraft client and show informations while playing in a singleplayer world. It works in single and multiplayer.

The corresponding app can display serverinformation, like online players or motd, live player infomation, like health and foodlevel, worldinformation, like weather or time.
The most interestings parts are probably the following two:
The mod allows you to mark blocks with a label, then the app tells you whether this block is powered by redstone or not. So you can for example see on your phone if there are items in your mob farm or if your quarry (Buildcraft) is finished or whatever you can think of. You are also able to control marked levers in the world via your device.
The second most interesting feature is the inventory viewer, which allows you to view your inventory on your mobile device (At the moment it only shows your items in textform, and Android only).
There also will be support for other mods, as soon as more mods update to 1.8, suggestions welcome), currently there is only support for Thaumcraft, you can observe the aspect amount in nodes.

So all in all this app can be used in two situations:
1. While playing to show additional informations or to allow you to disable the ingame HUD.
2. While beeing not on the server to show informations about the world or other players.

I´m still working on the mod and the app, which doesn´t look great yet.
I would love to get some feedback or suggestions what I could improve or implement.

More information on the Minecraft Forum Thread:

Minecraft Forum

Android App:
Google Play
Amazon Store

Universal App:
http://maxanier.de/projects/mcss/universal.html

Wiki:
https://github.com/maxanier/MinecraftSecondScreenMod/wiki

Troubleshooting:

https://github.com/maxanier/MinecraftSecondScreenMod/wiki/Troubleshooting

More informations might be found on my twitter page:
https://twitter.com/maxanier