I have just a pretty major revision to collapsing categories, version 0.6.
It includes the following changes:

• Can have multiple instances of widgets, each with separate options
• No longer works as non-widget
• All options are stored in one database row
• Added more sorting options
• Added option to include or exclude certain categories
• Added option to expand certain categories by default
• Added option to have category names either link to category archive or to activate expanding and collapsing

I realized this morning that this new version requires WP 2.6+. I would like it to be compatible back to at least 2.5, if not 2.3. However, I can’t seem to get it to work. The problem seems to be in adding the widget. In 2.5- the widget appears and can be added, and options can be set. But when you click on “save changes”, the widget disappears from the list. I have tried replacing a 2.5 widgets.php file with a 2.6 one, and that worked. I have also checked the database, and the options are being updated. So it must be something to do with how the widgets get registered. I have spent several hours trying to figure it out, and need to get back to my day job. I would be very pleased if someone else figured it out. In the meantime, 0.5.10 will work with wordpress 2.3-2.6