ideone Ideone is an online compiler and debugging tool which allows youto compile source code and execute it online in more than 60 programming languages and share links to the code.
repl.it Like ideaone, can also install missing Python packages.
Onlilne GDB Online compiler and debugger forC, C++, Python, Java, PHP, Ruby, Perl,
Pascal, Fortran, Haskell, Objective-C, Assembly, HTML, CSS, JS, SQLite, Prolog, and more.
Coding Ground Advanced IDEs for C/C++, C#, GO, Java, PHP, Perl, Python, Octave/MATLOAB, R, Ruby, Scala, and compiler/interpreter for many other languages and frameworks.
JDOODLE very similar to ideone but a somewhat different set of languagse/compilers.
Dedicated: JSFiddle Online JavaScript, CSS, HTML or CoffeeScript code editor. Option to add various web frameworks. .NET Fiddle Online compiler for C#, VB.NET, and F# with .NET 4.5 & Roslyn 2.0. Option to install additional nuget packages.
(Cloud workspaces: https://codenvy.com/, https://codeanywhere.com/)
regex101 Online regex tester for PHP, PCRE, JavaScript and Python that highlights pattern and matches on the fly.
https://regexr.com/ Online regex tester with gut regex replace and explain functionality.
https://www.debuggex.com/ Can show neat expression diagrams (not active anymore).
https://regexper.com/ also shows expression diagrams.
https://myregextester.com/index.php Allows to optimize patterns (I guess this is based on this Perl's Regexp::Optimizer).
http://regexlib.com/ Regular Expression Library. Currently has 7601 expressions from 2492 contributors (most are subpar IMHO).
txt2re: regular expression generator (with code generator for perl php python java javascript coldfusion c c++ ruby vb vbscript j# c# c++.net vb.net)
http://regexstorm.net/tester
http://regexhero.net/tester/
http://www.beansoftware.com/Test-Net-Regular-Expressions/Split-String.aspx
http://java-regex-tester.appspot.com/
http://www.regexplanet.com/advanced/perl/index.html
http://www.regexe.com/
https://spannbaueradam.shinyapps.io/r_regex_tester/
http://rextester.com/l/r_online_compiler
http://www.analyticsmarket.com/freetools/regex-tester (and a general GA RE usage guide)
sre_yield: generate all values that can match a given regular expression, or count possible matches efficiently
Xeger: Java library for generating strings that match a specific regular expression.
PyXeger: Java library for generating strings that match a specific regular expression.
https://www.tutorialspoint.com/codingground.htm
PHP Sandbox: Allows to test PHP code on many php versions.
https://3v4l.org/ allows to run over 200 different PHP versions (including HHVM, etc.) in parallel - excellent for version compatibility tests
http://c9.io An online IDE that supports many languages (free for personal use).
http://phpfiddle.org/ A PHP online IDE with some extra resources like code search (saved, external code).
https://www.onlinegdb.com/online_python_debugger
EXREX is a command line tool and python module that generates all - or random - matching strings to a given regular expression and more.
http://rextester.com/l/perl_online_compiler
https://www.jdoodle.com/execute-perl-online
https://www.tutorialspoint.com/execute_perl_online.php
https://html-cleaner.com/ HTML/CSS/JS Cleaner
Query test: http://www.jsonquerytool.com/#/JavaScript
Sort array: https://aruljohn.com/jsonsort/
Generate C# classes from JSON: http://json2csharp.com/ (Desktop tool: http://jsonclassgenerator.codeplex.com/)
ConvertJson.com FromJson(to XML, HTML, CSV, SQL, YAML), ToJson, JsonTools (Lint, Viewer, Format, Path Analyzer)
http://sqlfiddle.com/ (MySQL, Oracle, PosgreSQL, SQLite, MS SQL 2014)
https://sqltest.net/ (MySQL, Oracle)
https://sqlizer.io/
https://sqlformat.org/ (online formatting service)
http://poorsql.com/ (Poor Man's SQL Formatter online, download for VS, SMSS, Np++, etc.).
https://codebeautify.org/
https://tohtml.com/ Syntax Highlither for varius languages (useful for HTML & RTF text)
http://www.fileformat.info/convert/text/charset.htm
Arul's network and other technical utilities: View your IP, country, ISP name, OS, browser/UA.
Convert IP address to hostname and vice versa, text case convert, UNIX timestamp to date, hash generator
https://aruljohn.com/
Record and share Linux/Mac terminal sessions: https://asciinema.org/