While doing presentations you can annotate PDF files and save the output to another PDF file. I have nightmares about lectures using OHP's where the lecturer spent ages trying to get a pen that worked and then you couldn't concentrate on what they were saying because you were too busy copying notes. Using a combination of Xournal and PDF documents you can streamline the process. Have your presentation done before hand, if you feel the need to clarify a concept or add other relevant notes you can do that while you're presenting, at the end save the file and upload it to the internet. This allows people viewing your presentation to focus on what you're saying instead of madly copying notes. The process is so simple and fast that a student could walk out of a lecture and immediately retrieve the file for printing or viewing. All that's required to do this is a computer and preferably a graphics tablet, although in a pinch a mouse could be used.
I know programs like this have been around for some time, but this is the nicest free one that I have used, so I thought I'd do a quick demo on how to use and configure Xournal.