As an example, a tool I find extremely valuable is Splashtop. There are different versions of the product, so it is worth giving it a try to see what works best for you.
I, for example, find the Splashtop Personal version fits my needs, but I also subscribe to their anywhere package which allows me to work across different networks.
The tool itself allows you to use the beamer in your classroom like an interactive whiteboard. It also frees you up such that you no longer have to be chained to your laptop.
It works by you installing what they call the Streamer on your PC, and then the Splashtop application, app, on your mobile device, ideally an iPad.
You can then start to remotely controlling your PC from the mobile device, and then by switching into “WhiteBoard” mode you are able to annotate your slides from the mobile device.
You can further make screen shots of the slides for distribution to your class later should you wish to.
All-in-all a great application, one that saves you time, and makes teaching interactive and fun.