This comes up surprisingly often. So often, in fact, that it may be worth considering whether it wouldn't be better to make example.com/categoryname the standard case, and to restrict the creation of page (or post) titles synonymous to category names, or at least provide an option to do so.
Anyway, a rather ugly (but functional on small-scale sites) workaround I use sometimes is to actually create a page at that location and to create a custom template for that page which basically says 'include posts from cat X'. Obviously, it doesn't change the output of any
wp_list_cats-like functions (i.e. those will still include the category prefix unless you hack classes.php as detailed here); but for a limited number of categories (and for those who don't like to hack core files), it works quite well.