I draw my slides in Xournal on slide sized pages, and then export to PDF to present them. I roll a few patches to the software to make it easier to make build slides, but otherwise it's pretty self-explanatory. (https://github.com/ezyang/xournal) I do the same thing for diagrams too. (http://blog.ezyang.com/2010/04/diagramming-in-xournal-and-gi...)