Apart from the HTML format that is offered by default in the Header bar ▸ Tools icon ▸ Export… dialog, the format drop-down allows you to choose TSV (Tab Separated Values, good for spreadsheets), XML, and iCal formats.
There is also a command-line interface (try hamster --help in a terminal), that is very useful for scripting.
For more advanced programmatic access, there is a D-Bus API, that can be introspected using tools like D-Feet and dbus-monitor.