Venny : online Venn diagram generator http://bioinfogp.cnb.csic.es/tools/venny/
Diff checker : https://www.diffchecker.com/diff
Count duplicates in a list : http://www.somacon.com/p568.php
Remove duplicate lines : http://textmechanic.com/text-tools/basic-text-tools/remove-duplicate-lines/
Imgur : online image hosting: https://imgur.com
Word file to markdown converter : https://word-to-markdown.herokuapp.com/
Rich text to markdown converter : http://markitdown.medusis.com/
GIF Maker : http://gifmaker.me/
Quick online map maker : http://www.arcgis.com/
Online test and showcase javascript code : http://jsfiddle.net
Code for putting the "fork me on github" ribbon on your project : https://github.com/blog/273-github-ribbons
Calculate distance between two lat-long points: http://www.movable-type.co.uk/scripts/latlong.html
Find lat-long of a point on google map: http://www.latlong.net/
Quickly map rows from a table with lat-long data, geocode addresses etc: https://www.easymapmaker.com/ , http://www.hamstermap.com
Python scripts on the web: http://www.skulpt.org/
JSON viewer and a good place to host and show jsons online: https://codebeautify.org/jsonviewer/
JSON tree viewer (spl for hierarchical JSON) : http://jsonviewer.stack.hu/
PlantText : text language for generating flow charts https://www.planttext.com/
Google Drive CMS : a Google Sheet with a custom menu option to publish the content of that spreadsheet to your site's API. https://www.drivecms.xyz/
Firebase : Lot of database related stuff. Can capture data coming in from GoogleDriveCMS or others; can be used for remotely pushing changes to apps, for authentication etc. https://firebase.google.com/
Netlify : CMS that can run off github pages https://www.netlifycms.org/
JSZip : http://stuk.github.io/jszip/ javascript library for creating, reading and editing .zip files in browser
epub.js : render .epub files in browser https://github.com/futurepress/epub.js/

Useful software, online platforms

Tabula : http://tabula.technology/ : Extract tabular data from PDF
OpenRefine : http://openrefine.org/ : Software to clean, refine messy data. Esp good for fixing spelling differences.
Shiny in R : Create web-based interactive visualizations directly from R, without needing javascript/html coding : https://shiny.rstudio.com/, Check out the gallery: https://shiny.rstudio.com/gallery/, host online: https://www.shinyapps.io/
Trinket : write, share, run Python and other code online https://trinket.io/
Jupyter Notebook : an open-source web application that allows you to create and share documents that contain live code, equations, visualizations and explanatory text. http://jupyter.org/. Check out an example