Hi,

I don't know which applications are considered essential, it may be a matter of taste. Most of the time I am using applications which are already there, webbrowser for example.

There is a wiki site on how to update the firmware
http://wiki.maemo.org/Updating_the_tablet_firmware

As long as your tablet is online you can search installable application from within the application manager or browse the maemo.org download site.

nicolai