Windows: Nitro Reader has been my favourite PDF software ever since it first launched last year: it's leaner and easier to update than Adobe Reader, doesn't feature ads or upgrade nags, and lets you edit and annotate PDF files. Version 2 of the package, just released, dumps the "beta" tag, speeds up PDF creation and adds browser integration so you can see PDF files directly in your browser.
I can't say I missed having a browser add-in from the original version: Chrome users already have a basic PDF viewer built-in, and having PDFs launch inside the actual Nitro software seemed better to me than doing so in a memory-hungry browser. However, if you do want the option, it now works with Chrome, Firefox and IE.
Beyond that, Nitro retains its familiar and useful tools: easy addition of signatures to documents, extraction of included images, and a Ribbon-style interface. For Windows users, it remains easily the top choice for free PDF software. Nitro Reader is a free download for Windows systems.