One of the best mods I've seen for Minecraft!
How to install this mod on Minecraft:
1) Make sure you've got the latest version of Minecraft available
2) Download the Portal Gun mod - http://tinyurl.com/65ok5wg
3) Download ModLoader - http://tinyurl.com/4k2ox8q
4) Download AudioMod - http://tinyurl.com/6fo822b
5) Open each of these files up with a program such as WinRAR.
6) Open up appdata, if you don't know how to do this, google it!
7) Browse to .minecraft/bin
8) Open up minecraft.jar with WinRAR or 7zip.
9) Drag and drop the necessary files into the jar (Be sure to read the text files that come with the mods, some go within specific folders)
10) Delete the META-INF folder in the jar.
11) Run Minecraft and you should be able to run the new mod.
If you have a few dollars spare be sure to donate to iChun so that he can improve upon this awesome mod!