We use JD Edwards from Oracle running off an iSeries server.
Excel is used a lot for putting together large journal entries and analysis. I also have to use PL/SQL quite a bit for pulling data out of various data warehouses we have setup. Those are currently on a cluster but we are working on moving them to a nice netezza system.
Quickbooks is ok for a small business. I've always worked in enterprise so I don't have much experience with it. I tried it at home once and though it sucked.