Have you ever tried to make something that requires long distances of redstone? If you do, this mod will help you alot. The transmitter block transfers a redstone signal wirelessly 200 blocks away. If you have a gravel block above or below it it will transmit for 500 blocks. This will save you a major amount of space 
A diamond receiver/transmitter will be able to transmit through walls and is able to transmit to all 6 directions. With Transmitter Mod you can save millions of redstone and save a heck lot of clicking if you are making something.
You can create weapons, mob trapping device, sos signals, and routing minecraft stations plus a lot more complex redstone grids!, I Really enjoyed playing around with Transmitter Mod (I could finally make my wireless cart station xD) Anyway I would recommend this just to have fun and play around with.
Many thanks to Djoslin for this mod.
Here is a nice little video showing this mods features 
How to Install Transmitter Mod for Minecraft 1.2.5
- Locate the minecraft.jar
- Open the minecraft.jar in a program like winrar and delete the META-INF folder.
- Download Risugami’s Modloader
- Download the Transmitter Mod & unzip Modloader and Transmitter Mod
- Copy all of the *.class files from the unzipped folders into your minecraft.jar.
- If an /imgz/ folder is in the unzipped folder, copy it over to minecraft.jar.
- Done!!
