I've used TurboTax Online to do my taxes for a number of years now, and quite like it. This year, I noticed one really well-designed feature: browser validation.
If you've never used the app, the client side is a very js/css/remoting heavy HTML interface. It's probably the best RIAs I've used, and it's been of that quality for several years. As you'd imagine, they have some pretty stringent browser requirements to make it all work.
After you sign in, they check your browser and list out all the officially supported options, broken down by OS, and OS version, so you can see at a glance that Windows Vista only supports IE7 and FF2, or that OSX 10.3 only supports FF 1.0.7+. The best feature of all, however, is at the bottom. There's a section that says "If you don't meet these specs, you can continue, but we won't help you if something doesn't work," and then a "Continue" button.
I happened to be using FF 1.5 on CentOS (Linux), but they are considerate enough to say "if you think it'll work, have at it", rather than blindly disallowing access if you don't have a supported combination. I've run into too many sites that just refuse to let you if you're not using an explicitly supported browser/OS, and it's stupid.
That's a big improvement over past years, where it would simply refuse to work if it wasn't a supported browser/platform.
Last year, it wouldn't even let me use IE 6, for the simple reason that I was running the 64bit edition of Windows XP. The platform detection didn't recognize the OS as supported, so it balked.
Hear hear! I've a loan with WellsFargo that won't let you in unless you are using IE or Netscape 7 on MacOSX and IE6 or 7 on Windows, and they state that they don't support "beta" browsers – well how about supporting the browser engine that Netscape 7 is based on? Or how about supporting the default browser that comes with OSX? The worst part is that they have a *very* simple site but these insane restrictions.