Google PageSpeed -> is an excellent way to measure your WordPress site's performance and receive clear, specific feedback on how to make improvements.
New Relic -> also provides a free library for measuring server performance in PHP and MySQL.
MySQL Tuner -> also quickly assesses and makes recommendations for optimizing your SQL configuration settings.
Linux Top -> provides a dashboard for your server's real time performance.
If using Varnish Cache, VarnishStat helps assess the performance aspects of your caching tools.
Zend Server Z-Ray -> provides X-Ray vision and deep insight into your WordPress PHP application
Additional for Performance Testing
Google Developers: PageSpeed Insights
Mozilla Firefox + Firebug + Hammerhead
Google Chrome + Google Speed Tracer
Gomez Instant Test Pro
Resource Expert Droid