Computational molecular biology actually. I work in a center that provides computing services and resources for mostly health science research. We have a pair of XServe clusters (dual powerpc-G5's and dual dual-core Xeon machines) and several dual dual-core or quad-core Xeon Linux machines. We'll be adding some dual quad-core Xeon XServes later this year. And we have something like 10-12TB of XServe RAID storage, looking to grow that to 30-40TB this year.
Even though I use OS X mainly, I spend at least as much time in it at a terminal as I do at the GUI (my "word processor" is usually just NEdit in an X terminal
).