I've been doing a fair bit of cross platform testing lately (mostly ad-hoc). I was hitting a heap of problems with earlier versions of Internet Explorer, and needed a way of running up older browser instances in the cloud to test webapps.
I started off using SauceLabs scout, but soon ran out of free minutes. I then started using Browserstack. For $19 per month I now have several hours of cross browser/platform testing (including testing responsive design on mobile).
It's pretty easy to use - pick operating system, pick browser version, select URL to browse to, it loads your site into a browser running on a cloud hosted virtual machine, and you simply interact with it through your web browser.
It has a very impressive range of platforms and browsers (all of the major ones are covered, including mobile platforms such as IOS and Android). It also fires the browsers up with the most common devtools installed (such as firebug, web developer toolbar etc).
Cool thing is I can cancel and reactivate the subscription whenever we need it (which suits the size of the project that I am working on - and budget).
Anyways - thought I would shout out into the ether and recommend Browserstack for ad-hoc cross browser/platform testing. http://www.browserstack.com/
For automated testing (and integration with CI tools) - SauceLabs looks like a great option. https://saucelabs.com/
For cloud based web performance testing I have had great success with Browsermob (which is now called Neustar). https://home.wpm.neustar.biz/