Collapsing Blue is the theme which WordPress.Mu uses right now.
It has collapsing side bar, quick-tags for comment form, resent posts and resent comments for the side bar.
The collapsing side bar is very convenient for weblogs with forums or photo gallery.
It opens more space for better image viewing and forum reading.
The implemented comment quick-tags makes it much easier for the page to open without loading additional plugin (if quick-tags are in a plugin).
The big advantage of this theme is that you can change its with by changing one only value in the header.php (read the READ_ME.TXT from the ZIP)
Collapsing Blue, Theme, WordPress Theme, Collapse, Sidebar