I've been enjoying this site and thought I'd introduce it.  It's essentially some kind of market, but rather then stocks, or commodities you trade, it's contracts associated to outcomes on future real-world outcomes

It's fun, and it's free.

A video on it

These are some of the rules

Article 3 : Playing The Game

- The Game consists of set of speculative markets in which players buy and sell contracts associated to outcomes on future real-world outcomes (e.g., sports games, elections, etc.). To invest in an outcome, players buy the associated contract. To divest from an outcome, players sell the associated contract.

- Contracts are valued between X$0 and X$100. While the outcome is undecided, supply and demand from the players determines the trading price (including the initial trading price) of each contract. But the ultimate final value of each contract depends on NF's determination of the actual outcome of the event concerned. For example, while a Miami vs Orlando football game is going on, the trading price of the "Miami will win" contract may vary. But once the game is over, those contracts may be cashed in for X$100 each if Miami has won, or nothing if Miami has lost. The trading pages for each market clearly display the criteria that will be used to set the ultimate final value of the contracts involved, depending on NF's determination of real-world outcomes.

- Contracts come in opposite pairs, associated to opposite outcomes. For example, to each contract "Miami will win" corresponds a contract "Orlando will win" (or, equivalently, "Miami will lose"). The values of opposite contracts are linked. When one trades for P, the other trades for 100-P. Together, two opposite contracts are always worth exactly X$100.

- To buy or sell contracts, players place "orders" on the market, specifying the number of contracts involved, and the preferred buying or selling price. For example: "Buy 10 contracts of 'Miami will win' at X$75 each".

- The best priced buy and sell orders are always available for all players to see, so they can aim their orders to buy or sell at other players's standing orders to sell or buy.

- For a transaction to actually happen, a player's order must match at least one order from another player. An order to buy contracts at price P will match any order to sell those contracts at price P or any order to buy opposite contracts at price 100-P. For example: an order to "buy 'Miami will win' at 75" will match an order to "sell 'Miami will win' at 75", but it will also match an order to "buy 'Miami will lose' at 25' (the two are logically and practically equivalent).

- An order that finds no match in the existing standing orders will itself be added to the list of publicly displayed standing orders, awaiting a match.

- Standing orders may be left standing indefinitely, until trading is actually halted, or they may be cancelled at any time by their owners.

- An order that finds a match may be partially or completely fulfilled. If the match or set of matches covers all the contracts specified in the order, then the order will be completely fulfilled. Otherwise, the unfulfilled part of the order will be sent back to the market as a standing order.

- Each market will go through 3 states: Open for trading (active), Trading halted, and Closed, in that order.

- While trading is open, orders to buy or sell may be submitted at any time. (Although NF shall not be held responsible for any technical or software failures that may prevent, slow or alter trading in any way, and for any length of time.)

- Once trading is halted, all standing orders are cancelled, and no new orders can be submitted.

- Finally, once NF's staff has determined the prevailing outcome, the market is closed and the corresponding contracts in player's portfolios are cashed in for X$ at a rate specified in advance for each possible outcome. For example, if Miami has won against Orlando, each "Miami will win" contract is cashed in for X$100, while "Orlando will win" contracts are cashed in for naught.

- NF is the sole rightfull judge of the appropriate time to halt and/or close markets. Its decisions as to which outcome actually prevailed in the real-world, and thus at which rate to cash in the contracts, is in its sole discretion, final, and without appeal.
 
- GAME OF SKILL. Although chance can never be entirely ruled out as a factor in human affairs and the determination of any real-world outcome, be it a sports competition, an unemployment rate, a box-office result, or a peace negociation, this speculation game is overwhelmingly based on skill, knowledge, and reasoning. For several reasons:

    * Players can choose the individual markets and topics on which to speculate. They are under no obligation to participate in any market in which their knowledge of the facts, or domain expertise is lacking. They can focus their speculation on those topics and markets where their expertise and knowledge of the facts reduces the element of chance to a minimum.
       
    * The game is played over the long term, and involves many varied topics and markets, so even if an element of chance may be present at the level of individual topics and markets, on the whole it is statistically reduced to nil as far as knowledgeable and motivated players are concerned.
       
    * Because this is a speculation game where players can buy and sell in real time to each other up to the point where the outcome is known, and sometimes beyond that point, in contrast to a betting game which does not allow one to retract his or her bet once it is placed, players exert a real and continuous measure of control over their investment strategies. Even if chance may affect outcomes in the real-world, it is primarily skill in anticipating the behavior of other players, and hunting for timely information about developments in the real-world that allows one to "buy low and sell high" in order to profit at the game. A typical case is one where a player learns of a real-world outcome before others, and profits from that advance knowledge with absolutely no risk whatsoever.
       
    * Prizes are given away at regularly scheduled auctions to those that make the highest bids with the X$ that they have earned in the speculative game. Winning an auction itself involves strategy with respect to other competing bidders. So not only is there a truly objective criterion for determining the winners (the highest bid), but it is also the case that one does not have to be the wealthiest player in order to win a prize.