Sauce Labs provides browser testing services for front end development and quality assurance. Their Scout service lets you test your public or private web app in any browser via a VM that's run in the cloud. Enter a URL to test, select an OS and browser, and a virtual machine runs in the web page so you can test in that configuration on demand. Scout can record and save screenshots and video of every session, and they can can be shared, embedded etc. with a dev team, which is nice for bug reporting. JIRA integration means those same videos and screenshots can be attached to JIRA incidents as well. They also let you run automated Selenium tests in the cloud with Sauce OnDemand, for those of you who work with QA automation tools.
I also liked what they describe in this blog entry discussing the security measures they take. Every session gets a fresh VM that’s never been used by anyone else, and at the end of your session, the VM and all its data is completely destroyed. Good for peace of mind if you're paranoid about such things.
Plans are tiered, starting with a free personal plan that gives you 45 minutes of testing per month. You can use it here.