Useful software resources for reviewing papers I have a paper for review and I would like to include comments (on clarifications/suggestions/errors) over specific paragraphs or statements. 

What are some ideal software resources that could help reviewers? I would prefer them to be Linux-based.
If you have access to the LaTeX source, there are a number of packages that will help you. Some include todonotes and fixme. 

However it's more likely that you have a PDF only. In that case, you need a PDF annotating package. A free cross-platform solution is Xournal, which runs on windows/linux (and maybe Mac). 

If you're on a iOS device, then Goodreader is a nice app that does annotations. 
There's always Adobe Acrobat as well. Both of these solutions are not free though. 

Update: (by @atiretoo)

One issue to be careful with providing comments on a pdf or other document is maintaining anonymity. Adobe Acrobat (and probably other software), automatically flags your annotations with information about you unless you are careful to remove that from the document before commenting.