Apparently no one's done a language definition for SuperH yet, so I spent about an hour tonight and wrote one for Notepad++. To use it, import the file by clicking on the menu icon crossed by a lightning bolt in the middle icon bar and hit import. Opening the file will allow Notepad++ to access it and it will automatically use the syntax highlighting for files with a ".sh3" extension. To use the file with other extensions after importing it, simply click on Languages > SuperH.
The syntax will highlight all SH-3 commands and all normal registers, as well as special registers, comments, and handles subsections (use "{" and "}"). Keep in mind that there must be a space between arguments in order for the arguments to highlight properly. For example:
Good:
mov.l @r1, r2
Bad:
mov.l @r1,r2
Hope it's useful.