PERFORMANCE MONITORING TOOLS
GOOGLE WEBMASTER TOOLS
Google Webmaster Tools helps in reporting your website page visibility, it also shows how Google crawls, indexes and ranks your site.
W3 TOTAL CACHE
W3 Total Cache is a plugin that improves the user experience of your WordPress site by improving the performance of the server and reducing the download times.
Stella is more like a developer's tool for debugging and monitoring your website and it's applications.