Ever since eToro was featured on TechCrunch and got dugg, I've been addicted to Forex trading with eToro. Actually, I've been playing this since yesterday. :)
As TechCrunch rightly points out, eToro's friendly interface makes the boring, serious world of Forex fun. At this stage, let me just state for the record that I'm not a professional Forex trader nor do I have any prior experience with Forex trading. I do however have the (very) basic knowledge of how currencies work.
With that in mind, I skipped the introductions and dived straight into the big bad world of Forex. Out of the 4 eToro games available, the first game I tried out was the Forex marathon. Just select the currency which you think would rise above the rest and sit back and relax as they race against one another. 
It was quite enjoyable to watch the Japanese sumo wrestler pitting himself against the American and European. As your selected currency rises/falls, the corresponding character will lead the pack or fall behind the rest. I'm sure many new traders will prefer this alternative representation to the standard graphs and charts as a single glance will tell you whether you made the right choice even if you're a layman.
However, Globe trader was the game that I excelled in. In Globe trader, you buy/sell currencies to make a profit. As the market fluctuates, you will hear a pinging sound, which was useful and sometimes annoying (it can be turned off).
Within an hour of risky trading, I made over $500. Unfortunately, my lucky streak came to an end and I ended up losing over $1000. This goes to show that the volatility of the Forex market is something that cannot be underestimated.
Although eToro's interface may seem very childish, it does actually provide vital details like the market spread, your current profit/loss and the ability to take profit or stop loss (profit/loss threshold). However, after my short trading experience, I wished it had more features like scheduling and the ability to carry out more consecutive trades. Nevertheless, eToro brings the much needed element of fun into the Forex trading world.
eToro runs on Windows XP and possibly Vista.