This will generate a site map using nested lists and h3 & h4 tags. Fairly rough, but serviceable.

{exp:query sql="SELECT blog_name, blog_title, blog_description FROM exp_weblogs"}
<h3><a href="{path={blog_name}}" title="{blog_description}">{blog_title}a></h3>
{exp:weblog:category_archive weblog="{blog_name} "style="nested" orderby="title"}
 {categories}<h4 id="{category_id}"><a href="{path={blog_name}/articles}" title="{category_description}">{category_name}a></h4>{/categories}
 {entry_titles}/detail}">{title}{/entry_titles}
{/exp:weblog:category_archive}
"separator">
{/exp:query}