Angry Birds has been making the rounds as a fantastic physics app, and I completely agree. I've come up with a few more uses in math for different age levels that I wanted to share.
Classroom practice/study time: I recently had a group of ESL students come over who were practicing English - specifically, a few sentences they were going to deliver for a powerpoint presentation. I pulled up Angry Birds and each student said their sentences and then got to fire one bird - as a class we worked through about 4 levels, everyone having a blast. This can work with any "step" type problem - someone comes up to put the next step on the board, recites a fact they learned, something from a book, etc.
Estimating: At all levels estimation is important, but specifically at the younger ones where it is taught. How long will the bird fly if it's this angle? How far? How high? Where will it land? Justification is great for these as well - "Why do you think that"?
Measurement: Projecting onto a whiteboard means you can measure off specific distances, and measurement is really at all levels.
Volume/Surface Area: Determining the volume or surface area of the blocks broken is great for the middle school age groups. Competitions between groups could include who can break the most surface area, volume, both, etc.
Just a few quick ones - any more? Please share!