Enable / Disable the Hidden Administrator account on Windows 7 or Vista
Microsoft Fix It Solution Center - It doesn’t get much easier than this: Select a device or program, select a problem area, then browse the tips and programs that Microsoft offers as a solution.
PCWorld - Articles seem to be kind of dated but still some good information in here
Microsoft at Home - Fix common PC problems - This guide from Microsoft covers the basics for keeping a healthy PC, plus tacks on some tips for locating problems. Reading through this should let you know if you have a software, hardware or connectivity issue and give you the smarts and apps to tackle it.
ESET Online Scanner - Free online Virus Scanner from ESET
Kaspersky Online Scanner - Free online Virus Scanner from Kaspersky (has been offline recently, good option if it is working again)
TrendMicro Housecall - Free online Virus Scanner from TrendMicro
Malwarebytes Anti-Malware (Download) - Very good malware scanner and removable tool. The official website is good, however they usually forward you to CNET’s Download.com which has started to do some very spyware-like practices themselves so I recommend downloading from FileHippo instead
SuperAntiSpyware (Download) - Another very good anti-malware scanner and removal tool
SIW - SIW, System Info. Shows hardware and software information, very handy
TuxRadar’s Guide - An older (2009) list but still some good info
YeoWorks Ubuntu Solutions - One Click Fix for common Ubuntu problems
Reddit list of Linux programs
Apple Discussions Support Forum - You can select the topic by OS, program, hardware or just type your problem into the search box.
CNET’s MacFixIt Feed - Tech blog CNET keeps tabs on all their OS X-related updates with the MacFixIt blog. Head here to search for updates, cool features and troubleshooting.
Quora - Apple Products - I’ve linked directly to the Apple-Products section, but really just punch in whatever you are having a problem with and search. Answers are provided by other users of the service, I don’t tend to use this all that much but others swear by it.
The MacRX Category on Cult of Mac - Cult of Mac keeps it simple: All Mac, all the time. Follow their troubleshooting-specific MacRX category to get updates on everything from good Mac maintenance and un-deleting files to how to fix start-up problems and get your Mac running fast.
Macworld Forums - Typical forum site with categories, search, etc.
macnn - Typical forum site with categories, search, etc.
reddit list of apps - large list of apps done by reddit users
fluid - Make standalone apps out of websites
opensource mac - Free Mac software
vienna - desktop rss reader
gimp on osx
OSX Daily Tips
SuperDuper - Drive Cloning software
Onyx - System diags
Disk Inventory Deluxe - shows space used
Reset Password on 10.4, 10.5 and 10.6
Reset Password on 10.7 and 10.8
Recover saved WIFI password
Adjusting Keyboard Backlight
Refurb.Me - Lets you track refurb deals on the Apple store
OS X Server Caching
Thread about OSX Server
Hidden Mavericks Pictures
Apple Video adapters and cables
This copy of the Install OS X Mavericks application can’t be verified Fix - http://blog.mconserv.net/2013/10/install-os-x-mavericks-application-cant.html