From Movable Type

Simplified example

 <mt:SetVarBlock name="tags"><mt:EntryTags glue=" OR "><mt:TagName></mt:EntryTags></mt:SetVarBlock>
 <ul>
   <mt:Entries tags="$tags">
       <li><a href="<mt:EntryPermalink>"><mt:EntryTitle></a></li>
   </mt:Entries>
 </ul>

In MT4, the above code (used in an entry context) would output a list of entries with the same tags as the current entries. Note that there is no "weighting" happening here, meaning that an entry that has 5 tags in common is not scored higher than an entry with only one tag in common, and entries will be sorted by date, not by "related-ness".

More complex but refined example

The version below includes a few tweaks, commentary and is formatted for readability:

<mt:Ignore> If the entry isn't tagged, we display nothing </mt:Ignore>
<mt:EntryIfTagged>
   <mt:Ignore>
       Store ID of the entry in context into the "current_entry" MT variable
   </mt:Ignore>
   <mt:EntryID setvar="current_entry">
   
   <mt:Ignore>
       Compile the current entry's tags into a boolean "OR" statement
       suitable for use in an mt:Entries "tag" attribute and store it in
       the related_tags MT variable.
       
       Private tags are also being output here in order to ensure all
       similarly tagged entries are retrieved but they are immediately
       stored in the variable and not displayed on the blog.
   </mt:Ignore>
   <mt:SetVarBlock name="related_tags">
       <mt:EntryTags include_private="1" glue=" OR ">
           <mt:Ignore>
               We use normalize here in case of tags with spaces
               or other problematic characters
           </mt:Ignore>
           <mt:TagName normalize="1">
       </mt:EntryTags>
   </mt:SetVarBlock>
   
   <mt:Ignore>
       Here we clean up the value of related_tags in order to get rid of
       all of the whitespace and make the boolean statement syntactically
       correct.
   </mt:Ignore>
   <mt:Var name="related_tags" strip_linefeeds="1" setvar="related_tags">
   
   <mt:SetVarBlock name="list_items">
       <mt:Ignore>
           Here we use our stored boolean statement containing
           of related tags
       </mt:Ignore>
      <mt:Entries tags="$related_tags" unique="1">
         <mt:Ignore> Skip listing the current entry </mt:Ignore>
         <mt:Unless tag="EntryID" eq="$current_entry">
            <li><a href="<mt:EntryPermalink />"><mt:EntryTitle /></a></li>
            <$mt:Var name="items_found" value="1"$>
         </mt:Unless>
      </mt:Entries>
   </mt:SetVarBlock>
   
   <mt:If name="items_found">
      <h3>Related Blog Entries</h3>
      <ul>
         <$mt:Var name="list_items"$>
      </ul>
   </mt:If>
</mt:EntryIfTagged>

Using this recipe for Pages

To use this recipe for Pages, simply convert the following:

• Entries => Pages
• EntryIfTagged => PageIfTagged
• EntryID => PageID
• EntryTags => PageTags
• EntryTitle => PageTitle
• EntryPermalink => PagePermalink