Jarnal is an open-source application for notetaking, sketching, keeping a journal, making a presentation, annotating a document - including pdf - or collaborating using a stylus, mouse or keyboard. It is similar to Microsoft Windows Journal and to the earlier Mimeo whiteboarding and Palm notepad applications. There is also a commercial knockoff of Jarnal called PDF Annotator - for $50 you can enjoy a subset of the capabilities that Jarnal provides for free.