One of the joys of MT 3.16 and above is the BigPAPI plugin, which allows other plugins to easily modify the Movable Type interface. Now instead of hacking templates, you just upload a plugin and you're good to go.
A light-hearted but much-longed-for example is the Smilies plugin which creates a "smilie bar" above your Entry Body and Extended Entry fields inside of MT. Just click on the smilie and it is inserted in your entry.
I hacked this plugin to use my own favored smilies, and I also had to adjust the paths in the code because my mt-static directory is on a different domain than my blog(s), but that was pretty simple to do.
Aside: This plugin seems to conflict a lot with other BigPAPI plugins. It did with NaughtyWordChars until the developer of that plugin updated it, and it does with RightFields 1.02. There is a much expanded, but shareware version of this plugin called Icon-o-matic that may work better for most people.