Designing a new plugin

Hi,
I need to add a new functionality to a blog, and I wonder which is the best way to do it:
This is a little form that should be located in every page, on the main sidebar.
At submitting the form, all data should be added to a new table.
The data from this table should be viewed from the backoffice
I am debating whether this whould be a widget or a plugin.
Since it is in the sidebar, it might look like a widget. However, the form details are static and need not to be modified in the back office.
It looks more like a regular plugin to me, and not a widget. However, it seems that there’s no official way to register a plugin within the sidebar. So I can’t think of a valid way to hook the form html to the sidebar automatically when I activate this plugin.
Any idea would be appreciated.
Many thanks in advance, and best regards,
Michal