I've used IEtester for years and found it very useful.However it's prone to crash a lot and some versions don't work depending on your set up (ie7 doesn't work in Win7 unless you are running Ie9 as normal - or something like that).
Despite many claims to the contrary its CSS rendering of each version is 99.9% accurate (excluding some of the transparency filters in some versions) and is a good way to check the look of a site but don't expect anything to work properly as far as any other dynamics are concerned. Also bear in mind you should only use it for local testing as its not really safe to offer it online as it will only be as secure as the version you are testing (I believe).
As I said I've used it for years to good effect but it does crash daily. A better alternative is to grab an old PC and install older versions on that. Most of us have an old pc in the cupboard that could be used just for testing.
Microsoft also allows free VMs to be downloaded and is probably the best option if you don't mind the extra work of getting it all together.