We don't have to do as much UI testing on websites that we develop because we start with cross-platform, cross-browser, standards-compliant, responsive designs. When we're asked to fix a client site or application, we almost invariably discover designs that are not so graceful. How can you check if your site looks as you expect it to across Chrome, Firefox, Safari, Internet Explorer?
The easy answer is browserling. Peteris Krumins is an excellent hacker who developed browserling to make UI testing so much easier. On the other hand, you could just go out and buy a whole bunch of hardware, install multiple operating systems on top of a virtualization layer while also setting up all the required browser/OS combinations.... Oh wait, that would be crazy! Go to browserling. Done.
Note: there is also a similar service at browsershots.org which is developed by Johann Rocholl of Germany (now a Google engineer in Seattle, WA). The browsershots service is ad based, so it's a bit more distracting if you're doing this casually. On the other hand, if you're doing this for a living, the project is open source which means that Johann has made it a lot easier to do that 'crazy' stuff mentioned above.
Thanks Peteris and Johann!