We do a compiler in C++. Probably as far from Web 2.0 as you can get (in this industry).
(Often true, but not necessarily)
Giving 10% performance improvement in some fields (games?) is worth a metric buttload of money.
As for who would pay for this, manually parsing any substantial XML vocabulary using DOM or SAX is a major PITA so there are always people willing to pay for automating this task.