Tools/Utilities: Time&date, 

Calculators:  Scientific Calculator,  Martindale's Calculators,  A Google search on"calculator" displays an online calculator and links to more online calculators.

Count Words, Chars, HTML Tags: Word Frequency Counter,  Word/Character Count (ads),  HTML Tag Count, 
Create HTML coding:  Get image data with Javascript (or use CacheViewer :: Add-ons for Firefox a Firefox addon),  Real-time HTML Editor,  Best Online HTML Editor – convert WYSIWYG ⇄ HTML Source,   Alphabetize Text Words (online),  Alphabetize a text or word list,  Number Each Line,  Reverse Text Generator,  sql - help to get rid of HTML special chars in database - Stack Overflow,  TextFixer.com (online tools to fix/change text and html),  HTML Character Encoder/Decoder Tool,  ASCII / EBCDIC Code Converter,  Unicode Text Converter,  Hex to Decimal Converter,  Decimal to Hex Converter

CSS/HTML syntax/validation:  HTML 4 Elements,  HTML 4 Attributes,  Global attributes - HTML5, 
Validation:  The W3C Markup Validation Service,  Web Validation,  Site check (untested),  W3C Link Checker (prefer Firefox "Pinger" extension),  W3C CSS Validator, 
CSS: Full property table,  Selectors,  Advanced CSS selectors,  CSS/Properties - W3C Wiki, 
related:  Dave Raggett's Intro to CSS,  Symbols

• Colors: HTML Color Picker,  HTML Color Names,  Safe Colors,  Color Palettes,  Lists of colors  (compact),  {{Desktop App: ColorPic}}

• Some HTML-5 NAV and header:  HTML5,  structural elements,  HTML/Elements/header,  nav element, 

File Conversions:  Convert Web Page to PDF - #1 Web to PDF Converter,  Convert Word Documents to Clean HTML,  Free Video Converter, Any Video Converter Freeware: Convert any video to MP4/WMV/MP3 for mobile devices,  How to Convert Files to a Different Format with Free Tools,  JSON to CSV Converter,  Online converter - convert video, images, audio and documents for free,  Online Image Converter,  PDF Converter - Convert to PDF Online Free, 

Maps and GPS:  GPS Coordinate Converter, Maps and Info,  GPS Visualizer: Convert GPS files to plain text or GPX,  (bookmarklets: [LatLong:])

Pattern Generation:  Create a New Pattern :: COLOURlovers,  Free Pattern Generators: Here's 8 Tools You Want To Bookmark - Hongkiat,  https://bgpatterns.com/,  Online Pattern Generator - 20 Tools For Designers,  Patternizer - Stripe Pattern Generator Tool,  Repper Patterns ' Online Demo, 

Screen Captures install to use: PicPick (free for personal use), Snipping Tools (Windows: SnippingTool.exe), Screen Capture and clipping on other devices. Copy & Paste

Sorting:  Sort Text Lines (bookmarklets: for sorting text and HTML entities)

Spelling and Grammar checking:  Online Text Correction.  I maintain a list of about 700 words in Evernote that I add to any new browser or software maintaining a dictionary.  For Firefox running on a big laptop I use the Firefox extension “After the Deadline - Spell and Grammar Checker” and it is probably best to leave such work for a computer that you would actually be doing updates from, but this might prove useful in some situations, such as a couple of paragraphs written into Evernote .

Text Compare:  Diff Checker,  Compare/options,  HTML Diff service (urls),  File Comparison Tools,  Repeated Words/Phrases (now requires free acct, not retested),

Weights & Measures:  free web metric conversion tools in javascript @ metric conversions . org,  Online Conversion - Weight and Mass Conversions, 

zipcode tools:  Find ZIP Codes Inside a Radius

Mixed tool collections: images⇆ASCII, business cards, color generator/picker,

More tools:  10 Excellent Tools for Responsive Web Design,  Cut & Paste HTML to JavaScript converter,  free web metric conversion tools in javascript @ metric conversions . org,  HTML to JavaScript convertor - Transforms blocks of HTML into JavaScript document.write() statements | Developer Tools | Accessify,  Tools And Plugins To Create A Responsive Website - 48 Items,  Javascript Calculator Generator

Sharing Desktop: Mikogo (Share desktop view with Windows / Mac / Linux desktop with others in real-time from their browser including viewable from mobile devices.  (presenter / user).)

Source: See David McRitchies tools list for updates.