Troll Tech did the Qt support for virtual drawing methods via QStyle.
Cristian Tibirna designed most of the new gradient effects, and Dirk A. Muller did a lot of optimization of KPixmapEffect.
The BlackBox team's neat diagonal gradient algorithm was cool and has been optimized and extended in concept to a lot of our gradients. In exchange BlackBox implemented a lot of our advanced gradients. These are cool people and I look forward to working with them more in the future.
Linux Mandrake deserves special mention for allowing me to work primarily on this and other KDE2 stuff. For those of you that don't know, Linux Mandrake is a RH compatible Linux distribution with much newer software and a widely recognized as superior KDE based desktop.
And of course, there is always me :) I did all the KDE classes for styles and themes (KStyle, KThemeBase, KThemeStyle, and KThemeCache), designed the KStep and Pillbox widget styles, did the Marble and System example themes, did the KStyle dynamic loading mechanism, extended the KDE2.0 standard color scheme, and do various small hacking all over the place for better style support (Qt, KDE Control Center, KApplication, etc...). I also started KPixmapEffect and guess I am the maintainer (although this should go to Cristian when he has time - he did most the work).
Not directly related to KStyle, I also do a lot of work on KWin styles and have done the System and Next-like style.
And of course, I wrote this thing ;-)