"Crocodoc is a free, online service that allows you to upload and markup documents, including PDF files. You can not actually edit the PDF file, but can add text, highlight, draw, and even strikeout text."
Math Open Reference is a free online reference for geometry teachers and students. Math Open Reference features animated and interactive drawings to demonstrate geometry terms and concepts. The table of contents on Math Open Reference is divided into four basic categories; plane geometry, coordinate geometry, solid geometry, and function explorer tools