Commission
Types: Action
Cost: $5
+2 Cards. Discard down to 5 cards in hand. You may play an Action from your hand once per card discarded.
Assuming my math is correct, the first Commission only discards one card, which makes Commission just a Lab minus, which makes Commission just a Lab minus in any set that doesn't contain Villages or Villagers. In other sets, however, it's kind of a Fugitive with a bonus, which bumps it up to just strong enough to cost $5. Good job on this one.
You only need to increase your hand size. You can do that with splitters\villagers and terminal draw, but you can also do it with sources of non-terminal draw like
Laboratory or
Caravan or
Stables. When you can increase your handsize, you can have this playing a card 6+ times, you just don't get it for free. It's a lot of fun.
Antechamber
Types: Action
Cost: $2
You may play an Action from your hand. You may reveal an Antechamber from your hand to play it again. If you didn't reveal one, gain a Silver.
The problem with making a minus version of a card that costs $4 or less is that the gap between $2 to $3, $3 to $4, and even $2 to $4 is actually pretty small power-wise. So as a result the minus card become either too close to the original to bother existing, or it becomes so much weaker that it's brokenly weak. This card is the latter.
Desert Throne
Types: Action, Duration
Cost: $3
You may set aside an Action card from your hand under this. At the start of your next turn, play it twice.
See my comment on Antechamber.
While I agree Antechamber is far too weak (playing it on a draw card, hoping to find an Antechamber, and then accidentally gaining a Silver, will feel awful. It should gain a card costing up to $3.), I think Desert Throne is a reasonable, if uninspired implementation of the Duration
Throne Room.
Thane
Types: Action
Cost: $5
Discard a card. Look through your discard pile. You may play an Action from it twice.
This is effectively a Throne Room that can search your discard pile as well as your hand, which is too strong compared to Throne Room to just cost $1 more. You don't even need to have an Action card in your hand to be able to use Thane effectively!
In a thinner deck it often won't have much of a discard pile to work with. If you're playing something other than an Action from hand, the discard becomes a draw-back. I think this is a pretty smartly designed card and perfectly well priced.
Signet Ring
Type: Treasure
Cost: $5
+1 Buy. When you play this, you may discard a card for +$3.
When you gain this, reveal cards from your deck until you reveal an Action card. Set it aside and then play it three times at the start of your next turn.
King's Court is one of my all-time favorite Dominion cards (when I have it, of course .) I also love Hinterlands, which I think this would strangely fit in with very well because of its on-gain effect. Even though Appeal does it too, an on-gain Throne-Room variant has somehow never even crossed my mind, and this implements it gloriously.
I'd like it a lot more if it top-decked the card to marry the top and bottom halves.
Solar
Types: Action
Cost: $3
Choose one: +2 Cards; or play an Action card from your hand, then if the player to your left does not reveal a copy of it, play it again.
... I agree that it would've been much better with +2 Cards as a consolation prize.
Strongly disagree. You often need
Throne Room-alikes as +Actions. Having Solar give +2 Cards when missing means that not only would you not get to play your card, but you'd draw 2 Cards dead in a deck that's likely full of Actions (as you need lots of Actions to get Solar to collide with them). This Solar lets you get a shot at those +Actions, but at least only costs 1 card when you miss.
EDIT: Expanded card details for clarity