A data projector lets you present material very effectively to a whole class but you’re tied to the laptop or PC to control the screen. If you have an interactive whiteboard, you can work at the board facing the class but this is an expensive option and is (usually) a fixed resource which can’t be moved from classroom to classroom.

Portable solutions which combine a short-throw projector with a board are available but are still very costly.
There are a number of alternatives to the interactive whiteboard that give varying degrees of flexibility in the classroom.
The ebeam is a small device which converts any white area into an interactive board. It can connect to the laptop by bluetooth (short-range wireless) or USB cable. It provides a portable interactive board solution. RM price here.
A similar device is the Mimio.
If you want to control the screen from anywhere in the classroom, there are two choices – RM classpad or gyromouse.

The RM Classpad (and other similar devices) is a small tablet that lets you control the screen using a pen on the screen. It can be used with or without an interactive board.

The gyromouse is great fun. You control what’s happening on the screen using a wireless mouse which you wave in the air to move around the screen. It’s much easier than it sounds. This is a cheap but very effective way of controlling your computer from anywhere in the classroom.
Video here. RM prices here.