Custom Sidebars, Fully Editable

I’m building a theme containing different sidebar content for each page. I’d like these sidebars to be fully editable by my client, just like the main page content is when editing a page.

Here’s an example of the layout (static html):

http://dadradesign.com/clients/opensash.com

As you can see, this sidebar contains h1 and h2 tags, paragraphs, links, etc.

Is there any plugin, custom field tweak, or other way to integrate that kind of editable functionality? I’ve found a lot of discussion about custom sidebars but nothing that seems to really address what I’m going for.

Thanks in advance!