Every few weeks, I like to browse the OpenOffice.org Extensions site to see what is available, and what people are using. New extensions that are both useful and well-designed seem to be getting few and far between. However, if you search patiently, you can still find extensions worth trying.
One of my most frequently used Firefox extensions is Session Manager, which allows you to save and open groups of tabs. Last Session is the OpenOffice.org equivalent of Session Manager, and, as someone who regularly works with multiple documents open, I have found it just as essential.
Last Session (not to be confused with Last Session Multi, a Windows-only variant by the same developer) installs two buttons -- one labeled "S" for "Saved" and one "L" for "Load" in the Standard Toolbar. As you exit, you can press the S button to save your current session. then, when you start OpenOffice.org again, you can press the L button to reload all the files you had opened the last time you were in the program. Last Session will open windows from all OpenOffice.org applications.
Unlike Session Manager or Last Session Multi, Last Session lacks the ability to store multiple sessions. It also does not open in OpenOffice.org's initial menu, requiring you to open at least one file before you can load the rest. Still, within these limitations, Last Session is an extension well worth having.