admin -> Presentation -> Theme Editor -> select theme to work on and all the theme’s files will be listed -> click on which file you want to edit (sidebar or sidebar.php).
Note for this to work, all the theme’s file must be CHMOD 666. After editing it would be smart to CHMOD back to 644 for security.
As for a plugin – no. GUI? The above is the best you get.
Widgets. It’s a plugin that let’s you adjust sidebars by dragging and dropping pieces of the sidebar into place.
http://automattic.com/code/widgets/
Note that if your theme is not widget capable, you’ll either have to get one that is or change the theme’s code slightly. Full instructions are on the above page.
