This page describes computer hardware that is widely used or hardware that I have found very useful.  I've listed the most widely used hardware first.

UPS- YOU SHOULD HAVE ONE.  Let me rephrase that -- YOU MUST HAVE ONE.  (For desktops, not laptops.)  Uninterruptable power supply -- it supplies ac power to your computer  for up to 30 minutes when (not if) the power fails.  A UPS IS NOT THE SAME AS A SURGE SURPRESSOR.  It prevents computer disasters sometimes caused by power failures.  Costco -- $100 (1000 Watt).  Plug Computer, Monitor, external hard drives, modem into the power-protection outlets, NOT THE SURGE-PROTECTION-ONLY OUTLETS.  Do not plug a laser printer ino a UPS (power-surge when the printer heater turns on is too much for it.)    If your router/modem is in another room, and does not have a built-in battery, I recommend you plug that into another (small) UPS.  You do not need a UPS for a laptop, unless you use an external, ac-powered hard drive with it.  You should test UPS once per month (newer UPSs automatically test themselves).  The batteries will need replacing every 1-2 years.  

Mouse-  Yes, a desktop has one already.  But if you have a laptop, I recommend you use an optical bluetooth mouse instead of the touchpad or joystick.  It's far more reliable.  Get a large mouse if you mostly use the laptop on a desk.  It's easier to use, but it does cost more to feed!

External hard drives-- provide extra storage if you collect video or music files, or want file copies you can take to friends. I have 2 4-TB Seagate Digital drives(Costco -- $140 each).  If it has an AC plug, plug it into your UPS.  If not, it's powered from your computer.  Get hard drives with USB 3.0, even if your present computer only has USB 2.0.  See USB 3.0 note at the bottom of this page.
 
Laser Multifunction device- prints, copies, sends/receives faxes, scans.  Much cheaper and takes up less space than separate devices.  I have a Brother that has worked well for years. Costco - $170)
 
Ink-jet printer- prints color photos equal to those you can purchase.  I have HP.  Color prints cost several times as much as laser black/white prints.  (And are much slower.)
 
Flash drive (thumb drive)- cheap portable storage 2, 4 GB or more.  Plugs into USB on front of most computers -- great for moving files between computers.  But you can now get a USB-powered hard drive for portability.

Microphones: Cheap ($20-$50) gaming headsets with mics plug into either 3.5 mm jacks, or USB 2.0 jacks. They work well for Dragon Dictation, but are inferior for recording videos for youtube.I recommend USB mics with built-in preamps for recording videos. (Mic + adjustible stand costs $100-200). Open playlist "How To Record Video Tutorials" , watch video "How To Install And Use Mics To Edit Videos".
 
IPAD/Iphone- many people have one or both of these.  I have IPAD (Mac Super Store -- SLO), surf the net from parking lots while my wife is shopping.  Also listen to many radio stations on SiriusXM (pay) and Pandora Radio (free).
 
Notebook/laptop computer-  Small or medium screen models can be used in economy airline seats.  Pc's are much cheaper than apple.  Laptops are portable, but desktops are much easier to use at home (large monitors, large keyboards, better speakers). I have Apple MacBook Pro, but usually run Windows 7 on it.)
 
Internet-accessible external hard drive-  Connects to the internet via your router.  Anyone you give permission to can access your files from anywhere there is internet.  Great for sharing large file collections or videos.  Costco used to sell a 2-TB Seagate GOFLEX Network Disk Drive, (I recently saw one remaining in their Goleta store). I can access mine from IPAD, laptop, or desktop (I played my music files from Rome).
 
WIFI extender-.  If you have trouble receiving WIFI throughout your house (or in your backyard), put this between your router and where you have trouble with your WIFI.  It just needs a power cord, then receives and "echos" your WIFI, extending the range.  You'll probably need (free) tech support to "configure" it. It may be simpler to get a high-powered router.

2nd Monitor: I STRONGLY recommend you connect a 2nd monitor to your computer. They are EXTREMELY USEFUL!!! See video "How To Connect A Second Monitor To Your Windows Computer" in Playlist: How To Set Up Your Computer Room
 
Internet security camera- Access over the internet via WIFI.  Indoor FOSCAM model costs only $100 (Herman Street -- Internet), and can pan/tilt by remote control.  Outdoor models (that move) sometimes cost as little as $160, but are usually more.   You'll probably need (free) tech help to configure it for the WIFI.
Note,  the (expensive) systems at Costco come with several cameras & DVR, need a cable from DVR to each camera.

Replacing computers-  Computers 4 or more years old are getting pretty out-of-date and should be replaced.  If you buy a really expensive computer with all the "bells and whistles", it WILL NOT last longer. 

  
Replacing keyboards-  Keyboards, believe it or not, don't work well when you spill wine, water, or coffee on them.  (I've killed 4 of them.)  When (not if) you replace your keyboard, shop around.  You can get a good keyboard for $15-$25, but can (needlessly) pay over $100. I recommend wired keyboard and mouse (not bluetooth) for desktops.

USB 3.0 Hub- New computers have this interface for external devices. It's as much as 10 times faster than USB 2.0. If you don't have it, I'd still recommend you buy external drives that do, for when you replace your computer.  For around $50, you can get a 7-port Hootoo USB 3.0 hub. (You will run out of USB connections on your computer, and the rear jacks are harder to access.)

Caddy Buddy- Hardware to use your Ipad as a teleprompter.  $160.  See picture below. Camera mounts behind a tilted mirror.  Ipad is in front of the mirror, facing up.  You look through the mirror into the camera lens, while reading the reflected text scrolling on the ipad.