Not small web applications, obviously, but things like spreadsheets, word processors, or any custom-built mission-critical core-business application you might be running. Here’s
Why You Can’t Pry IE6 Out Of Their Cold Dead Hands, which explains why some businesses are finding it so difficult to upgrade to modern browsers. “Our custom web-based software won’t run in modern browsers” features rather highly on the list.

Once upon a time, companies could run very old, outdated, obsolete systems and affect no one except themselves and their customers.  Now when they hang on to these systems, not only are they hobbling the entire web development / design industry, they’re also keeping live computers on the internet with well-known and well-exploited security holes.

Here’s a statistic I’d like to see: how many zombies on the Internet are running IE 6? I bet it’s a lot.