JSFiddle

The original. Run and share HTML/CSS/Javascript. Includes easy access to various Javascript libraries. Go to jsfiddle.net

.NET Fiddle

Run and share .NET Visual Basic and C# code. Currently supports console projects. Go to dotnetfiddle.net

sqlite3 Fiddle

Test your SQL in sqlite3 running inside the browser. Go to sqlite.org

SQL Fiddle

Play around with database, create your schema and test SQL. Supports number of databases, currently MySQL, Oracle, PostgreSQL, SQLite and MS SQL Server Go to sqlfiddle.com

Regex Fiddle

Confronted with a problem, you thought "I know, I'll use regular expressions."

Now you have two problems. refiddle to rescue! Go to regex101

Python Fiddle

Run and share Python code. Go to pythonfiddle.com

Tutorials Point

This one does not follow the official naming convention but compiles and runs code written in various languages. Checkout the supported languages from the top menu bar. The list is exhaustive, from Ada to Verilog via Brainf*ck and other more or less well known languages. Go to TutorialsPoint.com

R-Fiddle

Fiddling with statistics? Try out the R-Fiddle. Go to R-Fiddle.com

Interactive GCC compiler

This is not a regular fiddle. Instead of letting you execute c/c++ code the interactive compiler compiles your code and then show the assembler code. This lets you investigate what kind of machine code is generated for certain structures with specific compiler flags. Go to gcc.gobolt.org

ClojureScript REPL

ClojureScript is a programming language which compiles to Javascript (while regular Clojure targets JVM). Go to REPL

Logo Interpreter

Logo Interepreter by Joshua Bell runs logo programs directly in your browser. To get started, paste some example code from the right side to the field next to "Run" button and hit run. Go to Logo Interpreter

Brainfuck visualizer

Getting your next big thing out with Brainfuck can sometimes be a bit overwhelming. But no worries, with this tool you can easily step through the app. Brainfuck Debugger

FiddleSalad

Create your own web salad by mixing your choice of markup, style and programming logic. FiddleSalad supports various languages for each part. Go to FiddleSalad

Rust Play

Environment for writing and executing Rust programs. Go to Rust Play

ZK Fiddle

Play around with ZK Java framework for building web applications. Create UI, write Java code. Run and Test. Go to ZK Fiddle

Go Fiddle

Nice an clean environment for trying out Go, the statically typed language developed originally at Google. Go to Go Fiddle

Swift Playground

Test your Swift code with various versions of Swift. Jump to sandbox

oK

oK is a learning environment for the K programming language. In case your K-skills are a bit rusty, check out the oK documentation on Github. The thing is open source, so you can also run it on your own machine. oK

PHP Tester

Code goes left, results come right. Easy as that to test your PHP code. PHP Tester supports multiple versions from age old 5.x to (almost) latest and the greatest. Run PHP Tester

Scalastie

Practise your Scala skills in the interactive playground. This way to Scala

RubyFiddle

Fiddle with the Ruby language. Create snippets and execute them (also check the security note on top right corner - people are so clever!). Something totally not related to this Fiddle is "Hello Ruby" a book about programming aimed for kids by Linda Liukas. That makes a perfect gift! Write Ruby

PHP Fiddle

Quite comprehensive looking PHP playground with different frameworks and even debugger. Fiddle with PHP

C++ Shell

Play around with interactive C++ shell. Supports also stdin for your "Hello, [your name here]" fiddling purposes. C++ shell

Try ColdFusion

Back in the days ColdFusion was the thing.. In case it's still for you, this is something to try. Try ColdFusion

ColdFusion Fiddle

This one keeps it plain and simple. Your ColdFusion code on left, results on right. ColdFusion Fiddle