The basic idea: solitaire hold 'em poker. I've been on a bit of a playing card kick lately and the idea of a newish sort of solitaire poker flitted through my head a while back. I'm not too sure of the value of this particular game, but here goes the basic rules I came up with:
-1 deck of standard cards, jokers removed
-a handful of counters (I used 5, but 21 might be better)
1. Shuffle deck.
2. Deal out nine cards face down -- two for the player, two for the "house", five common cards.
3. Flip over player cards, then first three common cards. Decide whether to stay in; if not, pass a counter to the "house".
4. Flip over last two cards; stay in or pass counter.
5. Flip over house; winner gets a counter.
6. Player wins best-of-five.
My main concern in something like this is that it actually be sufficiently interesting as to be worth repeat plays. I've already come up with one refinement -- the "21 counters" game, where you start with 21 counters and push one into a pot for each stage of the deal (initial deal, flop, fourth street, river) if you wish to stay in, and pass the pot to the "house" if you don't. In this set, you win if you have over half your original chips after five hands. (Why five hands? 54 cards/9 cards per hand = 6 hands, but since there's no jokers there will be seven cards left at the bottom of the deck before reshuffling.) This particular ruleset can also be trivially converted into a blackjack-like banking game, but I'd bet someone somewhere has already come up with it.