If you read the title and clicked on this, you know what this is about, so let's just start.

Disclaimer:
Before you build any of these grinders, please keep in mind that some servers do not allow these, for various reasons such as creating lag, being unfair to other players, making it too easy to level, etc, etc. It is not my fault if you build any of these without first reading the rules and making sure that it's alright and get banned. So please, don't comment saying; "omg i band u suck."
Thank you.

Powerleveling:
Powerleveling is when you farm easy-to-level stats, like Excavation or Acrobatics, just so you can have a higher powerlevel. I do this on servers where it's more laid back, but, on faction servers, you only want to train the stats you'll use. So, obviously, you should strive for 500 swords as opposed to 500 herbalism. Some servers cap their mcmmo stats, so you may have a hard time doing this.

Acrobatics:
(I did some testing here, ouch.) The minimum block height you can fall from in vanilla Minecraft and die from is 24 blocks. So 23 is maximum you can survive. Now, a lot of servers do not allow auto-grinders, so let's steer away from those. With this information in mind, let's build our grinder.

1) Build a 23 block tower, any material is alright.
2) Build a ladder up the side.
3) For maximum efficiency, and if you are alright with this, have a cactus nearby so you can die and respawn with full health. You can always wait, however. 
4) Jump.
5) Repeat. I suggest trying to land in different places as if you land on the same spot multiple times you stop getting McMMO xp.

{Keep in mind that Feather Falling IV will not help, since it reduces fall damage, it also reduces the xp.}

Swords/Axes/Archery/Unarmed/Taming: 
This is a simple method that does not require a spawner. Simply patience. 

1) Build an X by X by 3 cave underground. Make it large, such as 50, 100, or even 500 blocks wide. 
2) Make it completely dark. 
3) Wait for mobs to spawn. They will spawn naturally, and thus give you mcmmo xp! Congrats, you built a darkroom!

{Xp for combat skills is gained by the damage you deal. Using Sharpness/Power V will increase this. However, Flame I and Fire Aspect II are DoT effects and thus give no xp.}

Repair/Fishing:
On servers with Kraken, this could be difficult. 

1) Build a small room filled with water, and build a window into it where no water will flow inside.
2) Make a redstone pulse that will push a piston out every once and a while.
3) Build an iron block (McMMO anvil) that will push out in front of the window.
4) Spam your use key, and you will auto-fish, and if you have a lot of string on you, it will constantly repair your fishing rod.

{This is illegal on several servers, use with permission.}

Alchemy:
You can't get xp for Alchemy by using hoppers, sadly. So you have to put the ingrediants in yourself. Set up a ton of brewing stands in a single row, and make a potion. Give it redstone, then glowstone, then redstone, etc. Walk rounds on the row interchanging the ingrediants. You'll keep getting xp.

{This one is perfectly legal but takes a lot of glowstone/redstone.}

Exacavation:
Do I honestly need to make a guide for this? Grab a shovel, break dirt.

Mining/Smelting:
Grab an Eff V Diamond Pickaxe and mine yourself a darkroom. While your mining goes up, since Smelting is a child stat, it will also go up.

Herbalism:
Build several rows of sugarcane, and run though breaking them at eye level.

Woodcutting:
Find a jungle, take a diamond axe, and tree fell. If you have a low Woodcutting stat, this can give up to 5 levels.

Phew. That was rather long.