This afternoon I've been playing with a real fun annotation tool (at least fun compared with Microsoft Word). It's called A.nnotate, and it's one of the simplest tools I've come across, letting you add small (or very large) notes, corrections, or scribblings that float on top of the document like little widgets.
By default the notes are anchored to where they've been put on the document, but you can simply move them about, or sort them on a one-page listing that will organize them by time or who wrote them.
Power users will get the most use of the small notes. You can re-color them one of 21 shades and give each one tags, either from a preselected list or by making your own. This is one of the simpler ways to organize corrections, things to delete, and additions, so whoever gets the document back can sort out what needs to be done and very easily turn it into a workflow.
In addition to Word docs and PDF files, the service works with entire Web pages. You can plug in any old URL and it will take a snapshot of the page in a similar fashion to Iterasi (review). These same notes will show up on a source list you maintain. Clicking on any of them will take you right to where you left the note on the saved page, which will stay the same even if the source content changes.
The service is free to use--to an extent. Each document you open costs credits. You get 150 free each month, and the standard document costs 5 credits a page. If you want to work on docs with others, and work on several larger, multipage documents, there are premium plans that expand the amount of credits you have at up to 50,000 per month.