Interactive Projectors are projectors that have a built-in system for detecting a pen. The projector, as usual, projects on a wall but now the wall can be used as an interactive surface and can be written to by a special pen. The pen provides feedback to the projector, which propagates it back to the computer connected to the projector. The software on the computer makes the changes to the display that the projector is projecting. This makes interactive projectors similar to interactive whiteboards - but without the board.

Here are a few companies, their interactive projectors and some of the specifications for comparison (the specifications are taken from the vendor's website):