O.k. this not exactly about online learning games, but its about learning languages in a playful and effective way. Rossetta Stone did a great job in developing this competently grammar free method, based on the ways children learn languages.
There are two ways to do it: Either you buy one of their CD based bundles (Mac and PC), or you subscribe to a 3, 6 or 12 month online subscription. I would recommend you the subscription, since all the rossetta stone products are rather expensive and I suppose a subscription would add some motivation to your studies. ( I have to learn perfect mandarin in 6 month, otherwise I have to pay again ;-)
Get your free demo here:
Rosetta Stone: Free Demo