Hi everyone,
the other day I did this:
I wanted to have the last post's username linked to its profile in the main index, and here it is:

in ubbthreads.php
find:
"</td><td width="15%" nowrap class="posttime">$time<br>$textby $Poster</td>";

and change it with:
"</td><td width="15%" nowrap class="darktable"><span class="forumtitle">$time<br>$textby <a href="{$config['phpurl']}/showprofile.php?User=$Poster&What=ubbthreads">$Poster</a></span></td>";

watch out for the class property because it depends on the look of your site.

Luke01