A list numbering thirty plus design, resource, presentation and cheat-sheet-like PDF documents — includes works from 37Signals, Dan Cederholm (Simplebits) and Cameron Moll to name but a few — be sure to check out the comments for additional link love.
“I have personally archived every one of these files due to the high quality of content. No matter what you call them; ebook, white paper, or resource they all contain information that no designer should go without.”