• Anna's archive : The largest open-library database of electronic files in many different languages.
• Project Gutenberg : A library of free electronic books.
• sci-hub : A database of freely accessible scientific articles.
• Internet Archive : A general database for freely accessible digital media, downloadable or consultable on site.
• Gallica : A database of digitised media particularly in art and literature.

• Codeberg : Non-profit collaboration platform providing Git hosting.
• Zenodo : Free hosting server of digitally identified objects (permanently hosted on internet).

• Digital Library of Mathematical Functions : The online version superseding the classical handbooks editions by Abramowitz and Stegun of mathematical functions.
• Univariate Distributions : An online chart of many standard univariate distributions for statistics.

Physics

• CODATA 2018 : Fundamental physical constants defined by the CODATA 2018 standard hosted on NIST.

Electron interaction with gaseous molecules/atoms

• NIFS : Survey of experimental, theoretical and evaluated (hybrid) electron-molecule cross sections reported in the literature hosted by the National (Japanese) Institute for Fusion Science.
• NIST - Elastic Scattering : Electron elastic cross sections scattering off atoms (in gases and solids) calculated with Dirac partial-wave calculations with a local interaction potential based on the optical model.
• NIST Electron-Impact Ionisation Cross sections : based on theoretical calculations from the semi-empirical Binary-Encounter-Bethe model.
• CCC database : Convergent Close-Coupling calculations of electron collision cross sections with alkali/ne atoms (one or two valence electrons).
• LXCat : Plasma Data Exchange Project for hosting data of electron and ion transport and interaction in low temperature plasmas. My database is hosted there!

Molecular and Atomic properties

• NIST Chemistry WebBook : Large database of molecular physical, chemical and thermodynamic properties from experimental or theoretical results published in the literature.
• NIST Atomic Spectra Database : Compilation of atomic lines and energy levels of excited states from a survey throughout the literature.
• Data Visualisation Catalogue : a descriptive table of various form of displaying data when thinking about producing figures.

• Regex : Testing regular expressions online ( regex cheatsheet)
• Unicode Table : The full table of unicode characters on one page (also gives the HTML and CSS codes!

• Etymonline : Online etymological dictionary of English.

• Esperanto Grammar : A website with tools for learning Esperanto and a concise outline of its grammar.
• Plena Ilustrita Vortaro : An online version of the 2020 monolingual dictionary of Esperanto with definitions and usage examples.
• Universala Esperanto Metodo : An old-fashioned progressive methodology, from a different epoch at the beginning of the 20th century, for learning esperanto based on illustrations designed thus for any language speaker.
• Esperanto12 : Modern methodology of learning Esperanto in 12 lessons with explanations followed by interactive exercises.
• Esperanto : Platform of the Universala Esperanto-Asocio promoting the spread and use of Esperanto throughout the world.

• Collatinus : Online lemmatiser and morphological analyser for Latin texts. Gathers various classical dictionaries (French, German and English) of the Latin language.
• Eulexis : Online Lemmatiser for ancient Greek texts. Gathers various classical dictionaries (French, German and English) of the Ancient Greek language.
• Woodhouse : English-Greek dictionary from 1910 hosted online by the Chicago University.
• Perseus Digital Library : Platform for searching over translated and original Ancient Greek and Latin literature with search engines in the word occurrence and dictionaries for English→Greek and Greek→English queries.

• MDBG.net : Dictionary of Chinese, accepts pinyin and character input. Shows stroke order, examples in sentences. Has advanced search method for words in Chinese. Very translator of English-Chinese but reasonably good for looking up Chinese words.
• Zi : Single character analysis dictionary. Shows origin and evolution of the character from earliest related records to contemporary form. Uses pictograms to illustrate the original meaning of the character's shape. Displays the pronunciations of other languages (Cantonese, Japanese, Korean, Vietnamese) that used the character. Presents definitions from various canonical dictionaries. Enlists morphologically related characters, extant and historical variants. Presents tools to compose new compounds of existing characters.
• Stroke Order : Animated stroke orders for learning how to write properly.
• Chinese Text Project : Largest database of Chinese classics in digital version, with English interpretation. Includes scriptures such as those of Laozi 老子，Confucius 孔子, ShiTao 石濤, etc.