Matt, the items which (generally) take up the most space on Tableau Server are Tableau Data Extracts. If you go to the "Admin" page ("Maintenance" in v7) and run the Space Usage report, you can see the objects (reports with extracts and extracts by themselves) which are taking up the most amount of space. You can also see how long it's been since the objects in question have been accessed by a user.
This report will give you a good idea of what to "target".
Thanks a lot for your answer. The Space Usage page is exactly what I was looking for. If I delete a workbook (and thus its associated extracts) from the server will that free up space on the C drive of the virtual machine running Tableau server? I noticed that the space on the C drive of the virtual machine was unchanged after I deleted a few workbooks from the server last week.
They will not be deleted immediately - we have a process which "sweeps up" on a regular basis. It also depends on WHERE Tableau Server was installed - it could be that the extracts are being stored on a different drive if Tableau was installed to that different drive.
Is there a way I can manually run this? On a non-prod box and I need to free up some space. Delete some unused Extracts but Tableau Server has not removed them yet.