Few more toys from darksteel:

[Pago]

Aside from the obvious trinisphere (i posted thread here because it would probably get moved anyways), I think darksteel offers alot more interesting toys

Darksteel Citadel
indestructable
tap: add    to your mana pool

possibly a worry-free way in affinity decks? Or is the mana curve too much of a problem?

Blinkmoth Nexus (a good complement for factory? I hate how manlands come into play tapped)
  : Add   to your mana pool
  : becomes a 1/1 flyer until end of turn
  : target blinkmoth gets +1/+1 until end of turn

Gemini Engine
When it attacks, put an attacking copy of gemini engine into play (its called a Twin token). Sacrifice it at the end of your turn
3/4

I think gemini might work, but its more of a cute card, since birthsign is a gemini. however, it is evenly priced (workshops) and it has a decent ability

[BrokenNut]

The problem with the Citadel being for Affinity, is that Affinity isn't the best of decks. So a simple indestructable colorless artifact land isn't going to give it the oomph the deck needs to be viable.

The nexus suffers from the problem that there is just simply better usually. Factory and 1 land = 3/3 on defense, this kills Jugernaughts as well as living through jackal pups. The nexus requires itself and 2 lands to become only a 2/2 on defense which isn't nearly as good. And for offense, while it does fly, who plays with creatures that block much anyways? The sacrifice in power really hurts, especially when to increase it the same, it requires more mana than the factories.
Compared to Conclcave it's about even. Hard to say really. Concalve comes into play tapped, but gives blue mana, but takes a blue and a colorless to animate. Its also a 2/1 flyer, so hits harder. The nexus doesnt come into play tapped, but only gives colorless. But it also only takes 1 colorless to animate, but swings for half as much damage. And that is usually a big problem in the fishy style decks, killing in a fast enough time frame. Im not so sure the nexus really would work that well for those decks. Perhaps a new manland deck may be built that can use all of the above, perhaps even others.

The real strength of the gemini engine is that the copy gains the orignals power/toughness. So if you giant growth, then swing, you get a 6/7 token as well which is REALLY strong. The only problem, workshop decks don't play with creature pumps. And the engine without them isn't that amazing for a 6 drop. The difference between 4 and 6 in something like TnT is very big. 4 drops hit turn 1, usually before mana drain mana. 4+ drops are usually "late" game and are there to just own the board against special mathcups. Karn against artifact mana, Triskelion against weenies, Plat Angel against combo. This is just an expensive beater, something the deck doesn't really need.

[Dr_Tongue]

On the topic of Darksteel this one caught my eye as a mechanic that could prove usefull in redundant cc creatures of a deck and an interesting effect with Nether Void was Aether Vial.

Aether Vial
1
Artifact-U
At the beginning of your upkeep you may put a charge counter on ~this~.
Tap, you may put a creature with converted mana cost equal to the number of charge counters on Aether Vial from your hand into play.

 Now if I could find enough 2/3cc creatures in a deck (that you cast for free) while using my 2/3 mana casting  "other spells" instead.
 Seems like it could be abuseable.
Eot creatures , dodges a Chalice by giving an alternate comes into play route(like, but not like ,the Illusionary Mask), no mana investment for a 3cc creature opening up more mana to other things(pump on a Nantuku/disruption,and the immunity to Nether Void makes me want to try it for kicks.

[DEA]

i saw that too
good with goblin welder

[Pago]

It could work in decks that have lots of 1cc critters, like slight and big O, but it might be limited in utility because the sole purpose of those decks (besides a few utility cards) is to churn out efficient weenies every turn.