Mirror
Treasure - $4
Worth $1
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While this is in play, when you gain a Victory card, each other player gains a Curse.
(It's a horrible treasure early game and it slows you down as much as other players unless cards like Great Hall are in the Kingdom).
This doesn't look unbalanced, but I worry a bit about it not being fun. In general, I don't think the "hurt everyone including yourself" card concept is going to be popular. Saboteur and Sea Hag get flak for not producing resources, but even they don't hurt you too. Taxman and Followers kinda sorta hurt you, but they help you more. Anyhow, this might be worth testing. It's not nearly as bad as Leveller.
Smelter
Action - $2
Reveal a card from your hand
If it's a(n):
Action: Gain a copy of it
Treasure: +$3
Victory Card: +3 Cards
Put the revealed card on top of your deck.
(Terminal silver that slows you down, inflexible Courtyard, or a gainer that either sacrifices the play of a non terminal or makes your terminal clash problems even worse)
Wow, you're really selling me on this card! But seriously, I think even though the card is flexible, the three options are probably too weak, even for a $2 card. I'd try a $5 version that gave +$4 and +4 Cards. Maybe beef up the Action-gaining too, but that might not be necessary. Duplicating a non-terminal by putting off playing it for a turn may be enough for that option.
Warden
Action - $4
+$1
Each other player may reveal a Curse from his hand. If nobody does, each other player gains a Curse.
Each player who didn't reveal a Curse draws a card.
Did you mean to word this such that you draw a card (because you didn't reveal a Curse)? Either way, you should clarify that.
Also, the card is…maybe political is the wrong word. It's bound to create grumbling about turn order. "Oh, the person to my right revealed a Curse? I guess that protects us all, so I can choose not to reveal and draw a card."
Contrarian
Action - $4
Draw 3 cards from the bottom of your deck
Put a card from your hand on top of your deck
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When you gain this, reverse the turn order (eg play passes to the right if it passed to the left before).
I never thought I'd like a card that drew from the bottom of the deck, but somehow the fact that you also put a card on top of your deck piques my interest.
The bottom half is a Bad Idea. For two reasons. First, it means that you have to actually track how many turns each player has taken in case you need the tiebreaker at the end of the game. More importantly, this can potentially lock one or more players out of the game for quite a few turns while two players ping-pong the turn order between them. I don't care that it's an on-gain effect and so must eventually end (barring Graverobber, etc.). It's still really not fun to sit there as the other two players take 10 turns in a row.
Leveller
Action/Attack - $5
+3 Cards
Discard any number of cards from your hand. Each other player discards the same number of cards, then draws until he has 3 cards in hand.
Donald tested an Attack that made each other player discard down to 2 cards then draw one. It mostly just obliterates your turn. Again, it doesn't matter that you also might be hurting yourself in order to pull off this devastating attack. It's just too punishing. Also, see my comments on Mirror.
Pyromaniac
Action - $6
Each player (including you) may put any number of cards from his hand on top of his deck.
Reveal cards from your deck until you reveal one costing $3 or more. Put it back, and trash the other revealed cards.
The interaction is…interesting. I wonder how often the other players will take advantage of it.
As for the Action, it seems weak for $6. It's basically an unlimited Chapel that might also trash a few more bad cards (and/or good cards costing $2). Cool with Squire, I guess. I'm just not sure it's different enough from Chapel in practice.
Tracker
Action/Attack - $5
+1 Action
Each other player discards a card, then reveals the top card of his deck and puts it into his hand. +1 card for every $ of the highest cost in coins on a card revealed this way.
Huh, seems neat. I would reword it: "Choose one of them; +1 Card per $ it costs." Maaaaaybe too powerful? I initially read it as "+$1 per $ it costs", which I like better.
EDIT: I just realized that Tracker essentially has the Leveller problem when you're already down to 3 cards in hand. Maybe not a deal breaker.