Useful websites that I commonly use:

For research:

Google Scholar - the best website for literature review

Journal/Author Name Estimator - Not sure which journal to submit your research on, use this tool, you can use this tool to find a suitable reviewer as well

Referencing

Mendeley - my recommendation

EndNote - I messed up once so not sure if I will use it anytime soon

Zotero - I have heard good things about it

RefWorks - Not bad

Lately, there are few other ones but these are the most commonly used.

Online Statistics:

Graphpad Prism - useful for quick stats

VassarStats - very good statistical software, I have used this a couple of times and is as accurate as any commercial software

Websites for general purpose:

• https://www.pdfescape.com/ - combine and edit pdf easily
• https://ctrlq.org/screenshots/ - take screenshot of any webpage
• https://goo.gl/ - shorten any url or convert to QR code
• http://e.ggtimer.com/ - online timer
• https://www.random.org/ - generates random numbers for your experiments, lottery tickets etc
• https://wetransfer.com/ - send your files upto 2GB for free
• http://etherpad.org/ - write, edit documents together
• https://faxzero.com/ - send free fax ( I haven't used it so not sure of this works)
• https://www.freeimages.com/ - search royalty free images on the web
• http://everytimezone.com/ - time zone made easy
• https://adblockplus.org/ - Block annoying ads on your browser
• http://www.boredbutton.com/ - website for when you are bored (prepare to be amazed)
• http://www.clipconverter.cc/ - convert movie clips to other format without hassle
• https://www.copypastecharacter.com/ - interesting characters that you won't find on your keyboard
• https://www.printfriendly.com/ - convert any webpage into a pdf while removing the clutter
• http://www.speedtest.net/ or https://fast.com/ - check your internet speed