Suppose (in Revolution) you have users with these usernames: user1, user2, user3. Create three chunks called user1, user2, user3 with the appropriate content for each user.
On the page where you want the chunk to appear, put this:
[[!$[[!+user.username]]]]
Using a snippet tag (with those same chunks) might be faster:
<?php
/* GetUserChunk snippet */
return $modx->getChunk($modx->user->get('username'));
BTW, in Revolution, if you have a chunk called (anonymous) (with the parentheses) it should be shown to users who are not logged in. If you don't have that chunk, they'll see nothing.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html