I've created a plug-in that I'm now using on my personal site to provide the category and archive drop-down lists. It's called “Daniel's DropDowns”, and it will create drop-downs with either a link or a button to go to the selection in the drop-down, and a CSS class can be specified for the button and the drop-down box.

daniels_dropdowns.txt - Daniel's DropDowns 1.0 (UPDATE: This functionality is now part of WordPress core.)

To install it, download the file, rename it “daniels_dropdowns.php”, and upload it to your /wp-content/plugins directory. Then, enable it, and add the template tags to your theme. (The documentation at the top of the file lists all the template tags provided, and the options that can be passed to them.)