At some point in time every computer user has looked at the files on his hard drive and said "where the heck did that come from, and what made it?" Sometimes the file in question is something someone emailed you, or something that was attached to a usenet post. You don't have to stay ignorant, though, because here's a list of file types.
If you need to look up any technology-related term, WhatIs?com probably has it listed. They also have great lists of emoticons and acronyms commonly used on the net. And here's a list of emoji.
The Wirecutter is a good place to find fairly unbiased reviews of products. I like theirs much better than the reviews found on the big media-related sites, as those reviews seem too tied to whoever buys the most advertising.
Kaspersky is my current go-to antivirus program, but any of the top contenders on the AV-Comparatives Reviews regular tests will do. I tend to stay away from Norton and McAfee, because I've seen too many computers infected while supposedly "protected" by them. No, I don't put antivirus software on Macs. I've yet to see a genuine virus on a Mac. (Adware, yes. No virii.) That's not to say that it can't happen. If you make a habit of downloading things from places other than the App Store, or worse, hanging out on torrenting sites and the like, you'd best install one of the top-rated Mac AV programs.
Malwarebytes is one of the very first things I install on any computer, Mac or PC. (Yes, there's a Mac version now!) If you do get any of that nasty adware, it will take care of it. The paid version will stay memory-resident and prevent adware. The free version will get rid of it if you get infected, but won't prevent it. (The free version is all that's available for Macs so far, unfortunately.)
How-to Geek has lots of general help articles for Windows, Macs, and mobile devices, written in very accessible language.
Can't find a phone number for that company? ContactHelp's got your back.
If you can't reach a site and you're not sure if the problem is on your end or theirs, check here: Is It Down Right Now?.
Roaring Apps is a great list of what applications are compatible with which versions of various operating systems.
Zamzar operates a free online file conversion service. They will convert almost any file format to almost any other, including some that I know of no other way free way to convert. I believe there's a paid version of their service, as well, but I've only used the free one.
Are you getting the internet speed you're paying for? SpeedOf.Me will help you find out, and you don't need Flash or Java to use it.
Trying to figure out how to delete yourself from a web service, and coming up blank? JustDelete.Me will help.