I always liked the idea of BrowserStack, but there's a gap in that it won't work with local servers that use NTLM authentication. Which is what I mostly create.
So lately I've taken to downloading the VMs from Modern.IE. Our minimum supported browser is IE9, so I download the Win7/IE9 VM, and also the Win7/IE10 VM. I get IE11 coverage from my Windows 8 host.
After downloading, I install Chocolatey and throw on Chrome, Firefox and Fiddler and it makes a great disposable web test VM. I should really make that into a BoxStarter script and see what else makes sense on there.
To come full circle, I manage my VM connections with RDCMan.