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.
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).
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.
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://html-cleaner.com/ HTML/CSS/JS Cleaner
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)
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
Record and share Linux/Mac terminal sessions: https://asciinema.org/