There are some interesting flash games recently. First of all there's this "MyMinicity" thing. It's kind of like building your own city, and the way to expand the city is to get other people to visit your city. With the growth of population, more problems (traffic jams, unemployment, etc.) will be introduced, and one has to make things better before increasing too much population. For example there will be links that will increase industry, transportation, and so on. My city is here....http://myminicity.ad-s.org/?id=FHead. Every click counts! (one IP one action per day.) After a while, it becomes harder to balance between different clicks because it is the visitors that can decide which link to click instead of yourself.
Another interesting one is the GoCrossCampus game. The main goal of the game is conquer the whole map. When joining the game, one can choose a force to join. Normally the map is your campus, so people just join their own dorms. The game is progressed by turns. One turn is about one day, and players can place troops, put commands, etc. That's all what a person should know to play. What makes things interesting is the coordination within and between forces. There could be spies who leak out battle plans for example. The weaker ones should also ally together to fight off larger forces. The game will go on until only one army is left, and would last a couple of months or so.
Both these web games require people to go there every day to check status or do things. The advertisement benefit is sure large. I have to admit that whoever made the game is clever.