Brand Your Portal's Icon
Here's a neat (albeit relatively trivial) trick: Ever notice when you go to some web sites, the default icon changes from the little IE or FireFox icon? You can do that for your portal too, and it's simple.
Just create a small icon (16x16), save it as "favicon.ico", and put it in the root directory of your portal's app server. Browsers will (most of the time) request this file and replace the icon automatically, even using it for bookmarks into the portal:
For more gory details on how all this works, and some other caveats, check out the Wikipedia article.