Karn.

[Pizzatog]

Tips, useful cards, lands and combos I could include are very much appreciated. Its a Karn deck, I'm trying to make it good for duels, but still able to hold its own in multiplayer.

// Lands
    1  Ancient Tomb
    1  Blasted Landscape
    1  Blinkmoth Nexus
    1  Blinkmoth Well
    1  Crystal Vein
    1  Darksteel Citadel
    1  Desert
    1  Deserted Temple
    1  Dust Bowl
    1  Ghost Quarter
    1  High Market
    1  Mikokoro, Center of the Sea
    1  Miren, the Moaning Well
    1  Mishra's Factory
    1  Mishra's Workshop
    1  Mouth of Ronom
    1  Mutavault
    1  Petrified Field
    1  Quicksand
    1  Rath's Edge
    1  Reliquary Tower
    1  Rishadan Port
    1  Scorched Ruins
    1  Springjack Pasture
    1  Stalking Stones
    1  Strip Mine
    1  Temple of the False God
    1  Tower of the Magistrate
    1  Urborg, Tomb of Yawgmoth
    1  Urza's Factory
    1  Urza's Mine
    1  Urza's Power Plant
    1  Urza's Tower
    1  Vesuva
    1  Wasteland
    1  Winding Canyons
    1  Zoetic Cavern

// Creatures
    1  Arcbound Reclaimer
    1  Copper Gnomes
    1  Darksteel Colossus
    1  Duplicant
    1  Jhoira's Toolbox
    1  Junk Diver
    1  Karn, Silver Golem
    1  Masticore
    1  Mycosynth Golem
    1  Myr Retriever
    1  Pentavus
    1  Platinum Angel
    1  Scarecrone
    1  Sundering Titan
    1  Triskelion
    1  Voltaic Construct

// Spells
    1  Basalt Monolith
    1  Blinkmoth Urn
    1  Bottled Cloister
    1  Candles of Leng
    1  Citanul Flute
    1  Clock of Omens
    1  Cloud Key
    1  Coalition Relic
    1  Crucible of Worlds
    1  Darksteel Forge
    1  Darksteel Ingot
    1  Doubling Cube
    1  Eternity Vessel
    1  Expedition Map
    1  Gilded Lotus
    1  Goblin Charbelcher
    1  Grim Monolith
    1  Helm of Possession
    1  Lightning Greaves
    1  Lotus Bloom
    1  Mana Vault
    1  Memory Jar
    1  Mind Stone
    1  Mind's Eye
    1  Mindslaver
    1  Mishra's Helix
    1  Mycosynth Lattice
    1  Nevinyrral's Disk
    1  Oblivion Stone
    1  Orochi Hatchery
    1  Phyrexian Furnace
    1  Planar Portal
    1  Portcullis
    1  Rings of Brighthearth
    1  Sculpting Steel
    1  Sensei's Divining Top
    1  Sol Ring
    1  Staff of Domination
    1  Sword of Fire and Ice
    1  Temporal Aperture
    1  Thran Dynamo
    1  Tower of Fortunes
    1  Umbral Mantle
    1  Urza's Blueprints
    1  Voltaic Key
    1  Welding Jar
    1  Worn Powerstone

[potato]

Part of me thinks it'd be better to take out some of those less-exciting nonbasics and put in snow-covered lands and scrying sheets as a cheap colorless card-drawing source. If you're even allowed to play basic lands since you're running karn, I don't know.

If you can, then you could put in fetches to improve top etc. And Scroll Rack. Scroll rack is a great card.

I'd also add in Ensnaring Bridge for combo with bottled cloister, and possibly Grafted Skullcap.

Silent Arbiter is also nifty and in the same vein. If you're gonna play Masticore, you might also like Razormane Masticore. Oh, and you may have forgotten Mana Crypt. Or maybe you intentionally left it out.

[Pizzatog]

I'm not allowed to add basics or snow covered lands. That's the big challenge with a Karn deck.

I don't want to run Scroll Rack without a reliable way to shuffle, like fetches.

All the other suggestions are great, and they're going into the deck.

Thanks!

[potato]

If you're going to run Phyrexian Furnace, might you want to add Scrabbling Claws and Relic of Progenitus?

[BC]

I am also putting together a Karn deck.  Your list gave me a few good new cards to add, so thanks.  Here are some cards I think you should consider.  I put * next to ones that are especially awesome.

Lands:
Gargoyle Castle
City of Traitors
Gemstone Caverns
Ghost Town
God's Eye, Gate to the Reikai
Maze of Ith

Creatures:
Arcbound Ravager
Stuffy Doll
Grim Poppet

Artifact Mana Acceleration: (I have a lot more of these than you do, with fewer lands.  Even the higher cost ones are good with Karn, since they can become medium-sized creatures).
Scuttlemutt
Manakin
Millikin
Sisay's Ring
Ur-Golem's Eye
Mana Crypt* (can't believe you don't have this)
Guardian Idol

Other Artifacts:
Arena of the Ancients*** (omg, so good)
Culling Scales
Forcefield
Phyrexian Portal*
Phyrexian Processor*
Pithing Needle
Powder Keg
Time Bomb
Scroll Rack
Storage Matrix
Storm Cauldron
Ward of Bones

[Harlequin]

Lotus Vale is an option too.

At that point you ~might~ want Candelabra of Twanos As you run a fair amount of lands that tap for multiple.   

Darksteel Pendant is kinda cool with karn, and Culling Scales could be good based on your curve.  It adds some nice cheap removal (and if you have DS Pendant or DS Ingot in play you can kinda control the board for anything under 3cc).

Myr Incubator and Myr Matrix arn't too bad either. 

I would take a 2nd look at anything indestructable that can be turned into an indestructable creature with Karn.