Of particular interest is the Apple Snow Leopard based Mac Manual, the computer networks guide, and the Idiot’s Guide To Photoshop.
You can check out the whole list here.
15 Really Useful & Free Technology Related How-To Guides
MakeUsOf.com, which according to their site is a daily blog that features cool websites, computer tips, and downloads that make you more productive, has a collection of really useful “How-To” guides in PDF format that you can freely download on them.