This is a tough one. I never did find a satisfactory use for Voltaic Key. At least, I haven't seen one that made its restriction look justified.
Sure, you can get extra mana out of your Grim Monolith or Metalworker, but that's not particularly amazing. Voltaic Key only seems to be really useful when it's untapping artifacts that have an activated ability which taps them. It was also fun with Phyrexian Colossus or Colossus of Sardia, but those seem sort of wimpy now, next to Darksteel Colossus. The best artifacts that have abilities which require a tap either give you mana or do something broken. The latter usually also require a sacrifice, making the Key useless with them...
Voltaic Key can untap...
Goblin Charbelcher
Null Brooch
Mana Vault
Dragon Blood
Sol Ring
Thran Dynamo
Candelabra of Tawnos
Disrupting Scepter
Belbe's Portal
Quicksilver Amulet
Urza's Blueprints
Grim Monolith
Phyrexian Processor
Millstone
Grindstone
Scroll Rack
Metalworker
...and tons of other stuff.
Voltaic Key is pretty versatile, but to truly maximize its power, I would use it in an artifact heavy deck. I haven't built such a deck, so I'm doing some guesswork here, but now that Metalworker is on my mind, I think that it might be cool to do the Metalworker/Staff of Domination combo I once heard about...
This deck would mostly be expensive stuff and artifact mana. If the deck can reliably get infinite mana, it doesn't matter what the kill condition is. At the moment I don't really have any ideas. Really it could just be Rod of Ruin: have three or more artifacts in your hand when you tap Metalworker, then use Staff of Domination with it to generate infinite mana, drop Rod of Ruin, and use it with Staff of Domination for infinite damage...
Cards that may or may not (some of them would seem sort of necessary) work well in such a deck...
Sol Ring
Mana Vault
Grim Monolith
Thran Dynamo
Metalworker
Staff of Domination
Voltaic Key
Thirst for Knowledge
Memory Jar
Tinker
Junk Diver
Fabricate
Brainstorm
Darksteel Colossus
Stroke of Genius
Seat of the Synod
Tolarian Academy
Null Brooch
Phyrexian Colossus
Mana Leak
City of Traitors
Ancient Tomb
Scorched Ruins
Trinisphere
Karn, Silver Golem
Platinum Angel
It's too bad Staff of Domination didn't come out until last year. I would have tried to build this deck back in the day--when I tried to build decks like this...
