I am currently addicted to
Mansion Impossible. It's a house market game where you have to try to buy/sell houses as fast as possible to get enough money to buy the 10mil mansion.
Definitely more interesting than
Url Adex, which is a stock market-like game where you can buy and sell URLs. Kathy, thought you might like it though -- if you don't have enough math stuff to do during your daily life anyway! Everyone else, link'll cite me as a referral if you use it, but it's honestly a very slow-moving game. Or maybe I just chose bad URLs.