A while back, Kongregate added “kreds” so that you could donate to developers and for other potential uses. You now see flash versions of the standard browser games where you can pay for bonuses, more turns, etc. There are some games with a free level with an option to buy the rest; when one had a badge added, that received just a bit of negative feedback.
The game of the week is Bloons Tower Defense 4, which has dived into microtransactions. It added a level grind, which you can pay to skip past. You can pay for a earning twice as much money in-game. You can pay for a variety of bonuses to your towers.
Feel free to discuss what you think of this as a development tactic. If you are one of the developers, please let us know how that is working out for you financially. I just wonder how it goes on a game where everything is done client side. If I want more in-game money, I can edit it in. It is not as though that can be violating the purity of the game or even particularly cheating when the developers will let me pay for the same privilege.