Few days ago I upgraded from WP 2.51 to WP 2.8. A day after the upgrade I was changing around the Widgets and I removed the Categories Widget from the sidebar. When the screen refreshed the Widget dissapeared and now it's gone.
It's not listed under Available Widgets or Inactive Widgets.
Things I've tried:
Deleted and re-uploaded the WP-Includes and WP-Admin folders
Switched back to Default Theme
Deactivated all plugins
Tried both IE and Firefox (and cleared the cache from both)