Found these on a reddit thread a while back. Might as well pass the information along. I do not know if any links are dead.
http://www.recoveryourlife.com/ • Help for those who struggle with self harm, suicidal thoughts, mental illness, eating disorders and abuse. Putting this at the top for importance.
Technology/Internet:
https://ninite.com/ • Select programs from a huge list, get a SINGLE installer that will download and install the latest version of everything you selected with no crapware and no input needed from you, this is an utter godsend.
https://www.pcpartpicker.com/ • Building a PC? Pick your parts here and track their price changes and where to get them cheapest.
http://www.haveibeenpwned.com/ • Find out if you have had your details compromised in a hacking attack no matter where it happened & get alerts in future if it happens. (This one is very useful. Them bastards got me)
http://isup.me/ • Want to know if you’re having internet issues of if the site you’re trying to get to is just down for everyone? Check here.
http://www.10minutemail.com/ • Get a random email address that self destructs in 10 minutes, perfect for signing up to websites you don’t want spam from.
https://www.mailinator.com/ • Type in an address “mailinator.com and open it’s inbox, any address. Everyone can use any address at any time. Good for signing up for things you don’t want spam from but you may need to use the address for later (bad for privacy so don’t use it if you don’t want other people seeing the mail you get).
http://cloudconvert.org/ • Convert between various file formats (including some pretty obscure ones)
Entertainment:
https://www.justwatch.com/ • Tell it what services you have access to (Netflix, Hulu, etc) and it will tell you where you can watch that film or show you have an urge to watch.
http://radio.garden/ • There’s a big map, drag the circle around and get live radio feeds from within that circle, globally, it’s awesome.
http://radiooooo.com/ • Pick a country, pick a decade and listen :)
http://www.omive.com/ • A TV & Film database that allows advanced searching (like searching for two actors & seeing their joint projects).
http://www.gnod.com/ • Recommends music books & movies based on other stuff you like.
http://www.offliberty.com/ • Download YouTube videos. I prefer Stacher.
Education:
http://www.paperrater.com/ • Online proofreading of your papers, checks your grammar, checks for plagiarism, etc.
https://www.desmos.com/ • Want to graph an equation? Do it here!
http://hinative.com/ • Learning a language? Ask native speakers questions here!
http://lang-8.com/ • Still learning another language? Get your writing corrected by native speakers here! (Edit: not allowing new accounts for over a year now. Sad.)
http://www.mathway.com/ • Solves maths & chemistry problems for you
http://www.westegg.com/inflation/ • If I had this amount of money in 1970 how much would that be the equivalent of today (this and many similar questions answered here)
http://www.slader.com/ • Find your textbook, it not only shows you the answers to the exercises but more importantly HOW you get to them.
http://www.debatepedia.org/ • Pros & Cons for many many topics! (Edit: Apparently not updated since 2011 though)
http://www.sheldonbrown.com/ • Ever wanted to learn how to repair your own bike? Learn here!
Other:
https://www.windytv.com/ • Real time weather map that is a thing of beauty.
http://www.exrx.net/ • A great site to help beginners learn about exercise.
http://www.myfridgefood.com/ • Tell it what’s in your fridge, it will tell you what you can make!
http://www.thistothat.com/ • I want to glue this to that, what do I use?
http://votesmart.org/ • Look up a politician and see their voting history.
http://usefulinterweb.com/ • A big list of useful websites
http://iscaliforniaonfire.com/ & http://whereiscaliforniaonfire.com/ • Well, is it? AND WHERE???
http://freecampsites.net/ • Listings of free camp sites, LOTS of them.
http://www.rhymezone.com/ • Type in a word and get other words that rhyme with it.
https://unsplash.com/ • Awesome free high resolution images that are stunningly beautiful.
http://prepaid-data-sim-card.wikia.com/wiki/Prepaid_SIM_with_data • A wiki with information on getting a prepay SIM for your mobile while travelling.