You know how your gadgets always break right after the warranty expires? Well, here's another truism: You'll never lose the user guide to your cell phone, printer, camera, or Wi-Fi router until you actually need it, and then it'll be nowhere in sight. That's why you should bookmark The Manuals, a search engine that claims to have access to almost 6 million product manuals.
The Manuals located all of the manuals I was looking for, including the Nikon D200, a Netgear Wireless-N router, HP LaserJet 1200, and a Siemens phone system. Most of the manuals are readily available in PDF or HTML form, but I noticed that some results pointed to torrents. If this is a service you think you'll use a lot, you can even install a search plug-in for the site in your copy of Internet Explorer 7 or 8.
If you're looking for something that The Manuals doesn't seem to have, don't forget that you're not out of options: Rick has written about other resources for finding lost manuals.