#SQL
General:
Program: Mimer SQL Validator
Online: FreeFormatter.com
###SQL Server / Transact-SQL
Online: SQL-Format.org
Natively: SET SHOWPLAN_XML
###MySQL
Online: MySQL Syntax Check
Natively: EXPLAIN
###PostgreSQL & PL/pgSQL
Plugin: pg-validator
Natively: EXPLAIN
###SQLite
Plugin: sqllogictest
Natively: EXPLAIN
###Oracle:
Natively: EXPLAIN