Manual talk:$wgExtraSubtitle

This didn't work for me (using 1.6.9). After much experimentation I eventually found that the Mediawiki:Tagline did the job, but I also had to edit the monobook main.css file which did say display: none; }
 * 1) siteSub {

to change to something like:

/*display: none;*/ font-size: 84%; font-weight: normal; line-height: 1.2em; color: #7d7d7d; } Visctrix 22:29, 6 March 2007 (UTC)
 * 1) siteSub {