While I’ve never been one to write top n things posts, there are three PDF tools I use routinely that are pretty awesome.
The most generally applicable is PDF Creator, which lets you print to PDF on any Windows system. It uses the magic of Open Source ™ internally, essentially ghostscript, to make it happen.
The second is, again for Windows, PDF-XChange Viewer. It’s free as in beer tool by Tracker Software Products. It will allow you to add annotations to existing PDF files, line art, text, and so forth. Useful for marking up PDF files with copy edits, perhaps when working with a desktop publisher or artwork designer.
The third is, oddly enough, an OpenOffice.org plugin. While I find OOo entirely lacking for many — most — tasks, Sun’s PDF import extension allows you to import PDF files and make changes. It’s awesome, as long as you don’t need to modify the original PDF file. The changes must be saved as part of an OOo draw file. Great for changes to PDF forms that are outdated, like a HSA employer contribution form for the prior year.
The second and third I use sparingly, but have found both essential when needed.