Below is simply a list of links that I find useful while debugging/developing my applications and though you may find useful as well:
JSON Formatter: Sometimes you are given massive unstructured JSON files, this website formats them so they are much easier to manage.
XML Formatter: Similar to JSON Formatter but for XML files.
SQL Formatter: Similar to XML Formatter but for SQL statements
DiffNow: Plenty of plugins for Sublime Text or VIM that do this however if you do not want to get those plugins this is a quick website to find the differences between two files.
Text Generator: To generate dummy text for testing purposes and don’t want to install any plugins to generate them this website works well.
JAVA Escape/Unescaper: To output either escaped strings for java or unescaped strings.
This list will continue to grow as continue developing but wanted a place holder for the links. Hope this helps in your projects.