Website Image Speed Test - Beautiful & easy free analysis of the images on your page
browserling - Live interactive cross-browser testing from your browser.
Checkbot - Checkbot will boost the SEO, speed and security of your site by checking you're following 50+ web best practices. Browser extension with free and paid plans.
BrowserStack - BrowserStack is a software testing platform for developers to comprehensively test websites and mobile applications for quality.
Smaller - Make your websites load faster
Browsershots - Browsershots makes screenshots of your web design in different browsers.