I was a bit surprised that, at least in D6, the organic groups module doesn't supply a block to show a user's group memberships. We had to display such a block on a client site recently and defined the box ourself. First, define a new block via hook_block():

   1:<?php
   2:function forumone_block($op = 'list', $delta = 0, $edit = array())
   3:{
   4:    switch ($op) {
   5:        case 'list':
   6:            $blocks[0]['info'] = t("OG: Show a profile's group membership.");
   7:
   8:            return $blocks;
   9:        break;
  10:
  11:        case 'view':
  12:            switch ($delta)
  13:            {
  14:                case 0:
  15:                    $block['subject'] = t('My Groups');
  16:                    $block['content'] = forumone_users_og();
  17:                break;
  18:            }
  19:            return $block;
  20:        break;
  21:    }
  22:}

The forumone_users_og function takes advantage of the fact that a user's group memberships is a property of the object returned by user_load(). This block assumes that the uid comes from the url of the current page.

   1:<?php
   2:function forumone_users_og()
   3:{
   4:    // should be the profile the current user is looking at
   5:    $uid = (int) arg(1);
   6:    if ($uid)
   7:    {
   8:        $usernode = user_load(array('uid' => $uid));
   9:
  10:        if (isset($usernode->og_groups))
  11:        {
  12:            foreach ($usernode->og_groups as $ogid => $og)
  13:            {
  14:                $items[$og->nid]['data'] = l($og['title'], 'node/' . $og['nid']);
  15:            }
  16:            return theme('item_list', $items);
  17:        }
  18:    }
  19:}
  20: