Here's just a list of various card ideas that have been percolating around my brain for some time. Since I'm unlikely to ever put this together into the beautiful card mockups that some of you make, I decided to put the ideas out on the forum for others to use/laugh at as you see fit 
Cathedral
+2 Cards
You may trash a Treasure card from your hand. If you do, +1VP.
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When you buy this, you may overpay. +1 VP for each coin you overpaid.
$4+ Action
My kids always want their Treasures to count for something at the end of the game. I don't think that's such a great idea but here is a card to turn Treasures into Victory Point chips along with an overpay option for more chips.
Interesting. Random Trivia fact: The immediate effect is less cost effective than a Colony until you pay at least $44 for a single one (you get +40 VP, which is equal to buying 4 Colonies, which costs $44). I've never had that much in a single turn, though.
P.S.: This is not a complaint, this is just a random fact.
Cauldron
+2 Cards
Put this on your Tavern mat along with any number of cards from your hand.
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At the start of your turn, you may call this to trash the cards you set aside with it, gain a card with cost exactly equal to the total cost in coins of the trashed cards, and put the gained card into your hand.
$5P Action - Reserve
A Forge variant that requires a Potion and a turn on the Tavern Mat.
Interesting. Cons: Requires a Potion and has a delay on the Forge effect. Pros: Gives +2 Cards before choosing what to set aside, puts the gained card into your hand, and lets you do it whenever it would be most convenient (for example, if you set aside two Estates, you could hold off on calling this so that you can trash them into a Throne Room when you have a Grand Market).
Corrupt Official
+$2
Each other player does one of the following
- Discards a Treasure costing $5 or less.
- Reveals a Victory, Curse, or Ruins card from their hand and puts it on top of their deck
- Reveals a hand without any card meeting the previous 2 conditions
$5 Action - Attack
A corrupt Bureaucrat. Doesn't touch your Gold.
The problem with using "Treasure costing up to $5" is that most of the time, it will only discard Silvers and Coppers, but if there are Kingdom Treasures, such as Treasure Trove, this becomes about twice as powerful.
Innkeeper
Each other player with 4 or more cards in hand discards a Treasure card or reveals a hand with no Treasure cards.
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When you gain this, look through your discard pile. Reveal up to 2 Treasure cards from it and shuffle them into your deck.
$5 Action - Attack
Inspired by Inn and Le Miserable
Really lame. It's on-play effect is a $3-cost effect, and the on-buy effect is worse than Inn's (Inn lets you shuffle
any number of Actions into your deck). Inn's on-play effect is a $4-cost effect (Donald said that Inn originally had no on-gain effect and cost $4), which means that it's quite a bit stronger, but it costs the same. I would next to never buy this for $5.
Juggler
+1 Action
+$1
Look at the top 3 cards of your deck. Discard any number of them. Put the rest back on top in any order.
$3 Action
I have posted this one before. Fine, but not too exciting.
It is a fairly interesting card. Pretty weak, since on play, it's just an Action Copper. My gut reaction says it should be a $2-cost card.
Midden
You may look through your discard pile. Trash a card from it.
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When you trash this, gain a card from the trash.
$1 Action - Shelter
My attempt at a Shelter. Replaces Necropolis in your starting hand. Only trashes from discard pile to slow it down.
I'm assuming you can gain it back, since it turns into "a card from the trash" when you trash it? This is worse than Necropolis, I'd say, but it is obviously stronger than Hovel, since it does something. Nice for trashing cards w/ on-trash benefits. I think you should make the on-trash ability optional, though. In TfB or trasher-attack games, it's a benefit, but the rest of the time, it'll be a pretty major penalty.
Patron
Play another Action card from your hand. Add 1 to any vanilla +Card, +Action, +Buy, and +$ bonuses on the card.
$3 Action
Meant to be weaker than Throne Room. Works well with Market.
Yeah, I'd say it's worse than Throne Room most of the time. OP with Salvager, Apprentice, Storeroom, and Cellar, though. Probably better than King's Court with those four.
Pearl
$1
When you play this, reveal the bottom card of your deck. If it is a Treasure play it immediately; if it is an Action put it on top of your deck. Otherwise, discard it.
$4 Treasure
The Pearl Diver has to be after something, right?
This one's very interesting. At first I thought it was too weak, then I realized that Pearl is a Treasure. Oops.
Picket
Draw until you have 6 cards in hand. Put a card from your hand on top of your deck.
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When you gain a card, you may set this aside along with the gained card. Put the set aside cards into your discard pile after your next shuffle.
$2 Action - Reaction
A worse Watchtower.
Too much worse than Watchtower to only cost $1 less. This is probably just strong enough to cost $1.
Prophet
Gain an Action card costing up to $5 putting it on top of your deck. Each other player may gain a copy of it. If anyone does, +$2.
$5 Action
I'm not too sure about this concept. I toyed with the idea of giving +1 Card, +1 Action if anyone else decided to gain the card, but then I thought no one would.
The problem with this one is that it's too political. If even one opponent chooses to gain a copy and everybody else decides it's not worth giving you +$2, then those who don't gain a copy are basically being punished for somebody else's decision. I think it should just give +$2 flat out. Giving other players $5-cost cards is a pretty harsh penalty, especially because it's optional for them.
Ruined Garden
Worth 1VP for every 5 Ruins in your deck.
$0 Victory - Ruins
Ruined Lighthouse
+1 Action.
At the start of your next turn, +$1
$0 Action - Duration - Ruins
Spoiled Feast
You may gain a card costing up to $2.
$0 Action - Ruins
A few alternative Ruins
I've thought of a Spoiled Feast ruins before, but it let you trash itself in exchange for any Ruins type of your choice, as long as there is a copy of that particular ruins in the Supply.
Saint
Reveal the top 3 cards of your deck. Trash any revealed cards that cost $0 and put the rest back on top in any order. +$1 per card you put back. Each other player may trash a card costing $0 from their hand.
$5 Action
I wanted to make a card that could only trash Coppers and Curses but still be useful when those are gone.
Interesting. Pretty weak before you trash all your Coppers and Curses, but strong after that (Chapel is better at trashing).
Tenants
+1 Action
Reveal cards from your deck until you reveal an Action or Treasure card. Put all of the revealed card into your hand. You may discard an Estate. If you do, +$1.
$4 Action
Baron/Farming Village combination.
Weird.
Torch
Once per turn: You may look through your discard pile. Set aside a card from it. At the start of your next turn, put it into your hand.
$5 Event
Might be too strong.
I like it, but why Torch?
Treaty
Trash an Attack card from your hand. If you do, each other player discards an Attack card or reveals a hand with no Attack cards; Move your +$1 token to an Action supply pile that is not an Attack.
$6 Event
Might be too wonky.
So if there aren't any Attacks in the Supply, it's Training, otherwise, it's (mostly) better. The penalty that you can't put the +$1 token on Attacks doesn't make up for the attack part of the Event. Also, it should make you reveal a hand with no Attack cards to prove you can't trash one, or the Attack trashing portion should be optional.
Twins
$1
You may trash 2 cards from your hand having the same cost. If you do, +$ equal to the common cost of the trashed cards.
$5 Action
Trashing 2 Coppers gives you $1, 2 Estates gives you $3.
So it's Salvager that gives +$1 instead of +1 Buy, which is generally worse in combination with what it does, and makes you trash TWO good cards instead of one, but costs MORE. I would price this at $2. In fact, I probably wouldn't even buy this for that much.
Usurper
+1 Card
+1 Action
Reveal up to 4 cards from your deck. Put the last revealed card into your hand. Discard any remaining revealed cards.
$5 Action
A Laboratory-variant that lets you gamble on getting a better 2nd card.
Now, this one is probably your most interesting card. Good job on this one.
whenever you need it, and the advantage of not being able to draw it dead.