Useful online tools for StackOverflow

SQL

SQLFiddle
StackOverflow MSSQL
SQLize
SQLZoo

Regex

RegexPal (Generic)
Regexr (Generic)
Regex101 (Generic)
Rubular (Ruby)
LarsoLavtorvik (PHP)
MyRegexTester (.NET, PHP, JS, JAVA)

Programing Languages

IDEOne

Others

Table modeller
Text as table
Json Parser
XPath