I do it like this on my blog:
<div class="post-tags">
Posted In: <%- ("<a href='#{@getTagUrl(tag)}'>#{tag}</a>" for tag in @tags).join(', ') %>
</div>
Note, @getTagUrl comes from the docpad-plugin-tagging plugin. If you don't want hyperlinks to a page for each tag, you could simplify this to the following:
<div class="post-tags">
Posted In: <%- (tag for tag in @tags).join(', ') %>
</div>