One of the limits of WordPress seems to be not being able to tell how a user got to a post. As it stands now, as far as I can tell, you know if the user is looking at an archive or category query while actually in an archive or category listing, but once they click a permalink and go to a single post, you lose that state information.
If the user is looking at single.php I would love to know if he/she got there via an archive or category query, and if so which one.
This would let me do much more intelligent sidebar menu construction (e.g. expand and/or highlight the category or archive they were looking at) and it would also make more sensible breadcrumbs easier to implement.