PDF Annotator - Annotate, Edit and Comment PDF Files
PDF Annotator lets the user open any PDF file and add annotations, using the mouse or a Tablet PC pen, directly on the PDF file's pages. The annotated documents can then be saved directly back to PDF format.
http://www.ograhl.com/en/pdfannotator/
I've been deciding whether or not to get this software for my tablet PC with stylus and just came across this sale which, unfortunately, is going to end soon (less than 24 hours). It's still valid at the time of writing this and at this price (and exchange rate) I've bought a license for myself.
Hope you tablet PC or tablet users out there find this deal useful.
Mod: Changed link to unexpired link
For a free alternative, NitroPDF (currently Beta) allows you to leave notes, highlight text, fill interactive fields and annotations on the PDF document without creating any watermarks. Doesn't have tablet annotation support like this one though, but i figured that most people have laptops.
Try it out before purchasing this software.