Paper Trading and Online Trading Simulators

CME and Sungard Online Simulator
Through the use of simulated accounts which trade in real-time, the CME encourages you to experience Internet Trading of CME futures and options contracts.
CyberX2 and Cybertrader Pro Simulators
"Trade" in a simulated trading environment 24 hours a day, 7 days a week with the CyberX2 and CyberTrader Pro simulators. Without opening an account, and without risking any capital. It's free.

DaytradersUSA
Practice trading before you risk your capital. DayTradersU$A, in association with CyberCorp, is now offering an awesome stock trading simulator. Download.
Deal Station 2000
Foresight Exchange Prediction Market
Mock Trading
Sim-u-Trade
Trade futures or options without placing your funds at risk.
STOCK-TRAK
The internet's most comprehensive financial markets simulation available featuring: Stocks, Options, Futures, Mutual Funds, Bonds and International Stocks. 
StreetInsider Stock Contest
You start with a fantasy portfolio worth $100,000 and trade stocks as often as you would like. Free.
TradeComp International
Van Tharp's Secrets of the Masters 
This game totally de-emphasizes entry or "stock picking" and instead requires that you focus on the most important aspects of trading—Position Sizing™ and letting your profits run. Our game has five levels that get progressively more difficult to master. However, once you’ve mastered these principles, you’ll know you’ve mastered some of the key skills to trading success.
World Link Futures
Our futures and options Real Live Paper Trading® program is as real as you can get, complete with market orders, account statements and ticker symbols. You can trade all U.S.-based futures and options conveniently over the internet. It also comes with plenty of educational support starting with on-line examples of how to enter orders, how to read your account statement and answers to frequently asked questions