Play a "Connect-Four" style game, earning a game piece each time you answer an estimation question correctly. This game asks the players to estimate the answer to addition, multiplication, and percentage questions. Choose difficulty level, question types, and time limit. Estimator Four is one of the Interactivate assessment games.
http://www.shodor.org/interactivate/activities/EstimatorFour/

Here is a similar interactivity that doesn't have the Connect Four interface.

Estimator Quiz
http://www.shodor.org/interactivate/activities/EstimatorQuiz/

In this interactivity, you can choose 'Counting' where the students have to estimate the number of stars.

Estimator
http://www.shodor.org/interactivate/activities/Estimator/

More or Less Estimator
Is the number of objects more or less than the given estimate?
http://www.shodor.org/interactivate/activities/MoreOrLessEstimator/

The classic memory game, in a flipchart implementation. This version reviews mental percent questions. The questions are easily changed to suit any topic.

A visual way to teach percent operations is the 'TopHat' - which shows the relationship between the Part, the Percent and the Whole. This Java applet uses proportions to set up and solve percent problems.
http://nlvm.usu.edu/en/nav/frames_asid_160_g_1_t_1.html?open=activities&...

Mad Minute is a quick warm-up and review, typically taking one to two minutes. This MM reviews mental percent questions. You can use this as a template to create more Mad Minute pages. If you do, please submit them to the website!