The following plugins are very useful to WordPress developers. These may help you to make a standard and error-free theme and plugin.
- Developer.
- Debug Bar
- Query Monitor.
- Theme Check.
- User Switching.
- Regenerate Thumbnails.
- Log Deprecated Notice.
- RTL Tester, RTL Tester Mirror.
- Monster Widget.
- PHP Compatibility.
- SG Optimizer.
Thank you!