Company I work at still has their core HR, payroll, security, inventory, customer service, and other systems in a Visual FoxPro application backended by a SQL database.
Long back, I reverse-engineered the dBASE .DBF data format (with only a few hints from some mag or book, plus the use of a hex editor (DEBUG.com)), and then wrote programs in Turbo Pascal to read .DBF files and process them further, generate reports, etc., for a management / marketing game project that I worked on for an Indian conglomerate. Did this without Internet access. That was actually my first freelancing gig and first job. Heh.