crocodoc takes any PDF, Word document, PowerPoint presentation, image, or web page and lets you view and mark it up online. Documents can be shared with others, who can collaboratively highlight or strikeout text, add notes and comments, and make revisions. Whenever you and friends or colleagues are marking up the same document at the same time, you'll see each other's changes reflected in near real-time. Save your marked up version of your document as a PDF, in native file format or directly to your Google Docs account.
The free version of the software allows you to do everything except for the option of password-protecting documents, which are uploaded and viewed using bank-grade SSL encryption. The Pro version of the software costs $36 per year - a real bargain when you compare it to competing products.
Check it out: http://crocodoc.com/