Sites and apps similar to Assistiv Labs include BrowserStack. It's paid, but IETester is free. Other options are Browserling, LT Browser and Comparium. In total there are five Web Debuggers like Assistiv Labs for Windows, Linux and Mac.
BrowserStack is the world’s largest cloud platform for software testing, with 20,000+ real Android and iOS devices that allow developers to comprehensively test their websites and apps for functionality, performance, and visual appeal.
India
Browserling lets you quickly cross-browser test your websites in all the most popular browsers, such as Internet Explorer, Chrome, Safari, Opera and Firefox. We run the browsers in virtual machines and you get live access to them.
United States
Enables real-time debugging of websites on mobile and tablet views, supports 50+ device profiles, allows side-by-side testing, custom device setups, separate Dev Tools for each device, and screens for responsive design, all within a web-based platform.
United States
IETester is a free WebBrowser that allows you to have the rendering and javascript engines of IE11, IE10, IE9, IE8, IE7 IE 6 and IE5.5 on Windows 8 desktop, Windows 7, Vista and XP, as well as the installed IE in the same process.
United States
Comparium is a convenient web application to test any website on any operating system and browser combination.
