Web workers would have been a must have for a project I worked in a few years ago. I had to write a real-time video allocation algorithm that had dynamic hardware with extreme combination constraints, and varying encoding rules for audio and video (think HD, SD, Dolby, stereo). It also had to allow for changes without moving existing allocations (otherwise viewers TV programmes would glitch).

The back-end was an embedded device that only accepted XML and could not do the calculation.

I had to have this do the allocation in IE8! So I wrote a timer based allocation routine that would release the UI thread enough to not lock up the browser. It worked (sort of), but was ugly and took way too long!