MakeUseOf Guides
I've gotten so many good tips from MakeUseOf that it's one of my favorite sites; they find and review a lot of things I simply don't have time to look for.
They also have currently 38 free downloadable guides in PDF format on such varied topics as iTunes, iPhone, Linux, Photoshop, digital photography, online shopping, building a gaming PC, Twitter, creating professional documents in Microsoft Word, Twitter, and the Internet Guide for the Movie Addict. There's really something for almost everyone.
You might be interested in these other posts about resources I've learned of from MakeUseOf: Creative Commons, Semantic Search Engines, Functional Screensavers (PC and Mac), SAT Test Prep,