I have found a few great mods out there for the wonderful game Minecraft! I will have an overview of 4 of my favorite mods:
Mo' Creatures
SDK's Gun Mod
SSP Commands
ZombieCraft
I am not going to cover how to install mods, I may write a blog later.

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Mo' Creatures
This mod adds many new mobs, both friendly and hostile, to the game. A few friendlies are: horses (many kinds of horses), rabbits, birds, there are more that I cannot remember. Now, for hostile: wraith, shark, lion, werewolf (appears as survivor type character during day), wolves, lions, bears, and more!
Mo' Creatures

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SDK's Gun Mod
Adds guns/utilities items to the game! Of course the main feature is guns,which include: AK47, sniper rifle, shotgun, flamethrower, rocket/laser-guided rocked launcher, sentry gun (can use any gun). It has grenades: normal, stun, smoke, and sticky. It has a few special blocks like a nuke and cannon. Some utilities are: grappling hook, parachute, and nightvision goggles. That is just a small overview, there are a few more weps/utilities!
SDK's Gun Mod

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SSP Commands
It is just a mod that allows you to use LOT of commands like turn damage off, spawn items (usefull for playing with guns ), spawn mobs (even Mo'Creatures mobs), fly, and many more.
SSP Commands

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ZombieCraft
ZombieCraft is essentially a conversion of Nazi Zombies into Minecraft. It was recently released, so there are a few bugs. The mod comes with a map, the original Nazi Zombies map (Kino der Untoten?). Everything works very well for the game, and it gets pretty intense. SDK's gun mod is intigrated into it, so there are guns (even a raygun). There are also perk machines, ranging from juggernog to support sode (spawns friendly AI). Mapmaker coming soon!
ZombieCraft
You must sign up to download ZombieCraft