User Interface (UI) advice - see learnui.design, e.g.

https://learnui.design/tools/data-color-picker.html#palette

w3schools.com - html tutorials, css coding, colour codes, etc.

Everything needed (except our time) for building or redesigning a website

Digital Colour Meters - These measure the colour we can see on any selected area or point on your screen, and then give us the html code value that specifies the selected colour. One of these is provided as part of MacOS  (see https://support.apple.com/guide/digital-color-meter/welcome/mac ). Colourzilla has Firefox and Chrome Browser extenstions that do similar things on any computer (see https://www.colorzilla.com/ ).

These should be very useful for website colour design. I can also imagine using a digital colour meter to objectively measure colour and tone differences in a series of photographs for scientific measurement of anything that has been photographed in a standardised way. Or it could be used to match the colour of digitalised slide photo with the colour of the same object photographed with a digital camera, or to adjust photos that have a standard colour chart included in the photos.

How many different file formats exist for digital information?

It is a jungle out there. Some formats are extinct, many are alive, and many are evolving.

The Library of Congress Digital Formats website has a comprehensive catalogue with descriptions of old and new file formats. The information is provided to help librarians and archivists manage digital collections. Look for their Format Descriptions tab.

Currency conversion is useful when negotiating prices, or buying books online etc. Here are three commonly used services:

Oanda
XE
X-rates
Yahoo

Less well known is the following equation:

Volunteer x Worried Writer x Sincere Effort = Mutual Satisfaction (a universal social currency). Bananas also deserve respect

United Nations list of countries and areas for statistical purposes ("M49 standard"): http://unstats.un.org/unsd/methods/m49/m49alpha.htm . 

The International Organization for Standardization displays 248 official short names for countries, and short identification codes for each country .

The World Gazetteer by Stefan Helders is a rich source of information on places and geographical data.

Maps of Net is a huge aggregation site for maps of the world, with comprehensive destination gazetteer.

The AuthaGraph world map shows the true areas of ocean and land in a rectangular layout.

I often need to look up international country codes. The following sites are useful:

AT&T Internet Service

CountryCallingCodes .com

StudyAbroad .com

World Telephone Numbering Guide

Check the time in another country when planning a phone call, online meeting, or chat. These sites typically carry a lot of advertising. Sorry, that's the business model, but the sites are useful.

Everytimezone  - an interactive graphical interface to compare your time across time zones.

The Time Now  - plan meetings across multiple time zones, and more.

The World Clock  - comprehensive with many options.

Time Zone Map - world time zones, dates, and times.

World Clock Meeting Planner  - find the best time for an online conference across time zones.

Time and Date - all the above features, including a very useful Date Calculator, which can show the date for a given number of days, years, etc after or before a given date.

Alternatively, watch the stars and planets and use their position to navigate in time and space. See the Stellarium

The Science Dictionary - A "specialized science search engine that queries billions of the web's top science pages for scientific information"

Online Life Science Dictionary - based in Japan.

World of Science - "Online Science for All" b ased in India.

Babylon Science & Technology - "The World's Best Online Dictionary"

Biology Online - "Run by enthusiasts from all around the world"

Opposite Dictionary - "Find opposite of a word in seconds. Largest opposites dictionary"

Pronounciation Dictionary - "All the words in the world. Pronounced" based in Spain.

Science Dictionary - " The World's Largest Online Science Dictionary" b ased in the USA.

Astronomy Thesaurus - covers English, French, German, Italian, and Spanish.

CAB Thesaurus - multi-lingual coverage of life sciences, technology and social sciences.

ERIC (Educational Resources Information Center) Thesaurus - based in USA, covers bibliographic citations and digests, sponsored by the U.S. Department of Education.

Ethnographic Thesaurus - based in USA; covers folklore, ethnomusicology, ethnology, and related fields.

European Thesaurus on International Relations and Area Studies - based in Europe; covers international relations and area studies, and the languages English, French, German, Spanish, Italian, Polish, Czech, Croatian and Russian (English or German interface).

Getty Art & Architecture Thesaurus - based in USA; covers fine art, architecture, decorative art, and material culture.

Getty Thesaurus of Geographic Names - covers all nations of the world, including vernacular and historical names, coordinates, place types, etc.

Getty Union List of Artist Names - based in USA; covers artists and architects, including variant names, pseudonyms, and language variants.

LC Thesaurus for Graphic Material   - covers terms terms related to graphic materials.

Medical Subject Headings (MeSH) - covers medicine and health.

Public Health Language - based in the UK.

Thesaurus for the Social Sciences - based in Germany, covers terms in German - English - French.

UNESCO Thesaurus - covers "the fields of education, culture, natural sciences, social and human sciences, communication and information"

Visual Thesaurus - a commercial site that gives a sneak preview of a potnetially very useful service. The main service is not free unfortunately.

See also: Taxonomies & Controlled Vocabularies SIG of ASI .