<p>I'm interested to know what are the "must have" JSP tag libraries apart from JSTL. All I've found so far are</p>

<ul>
<li><a href="http://www.javaranch.com/journal/200601/Journal200601.jsp#a3" rel="nofollow noreferrer">ccc</a> - for accessing static constants in JSP (without scriptlet)</li>
<li><a href="http://displaytag.sourceforge.net/11/" rel="nofollow noreferrer">displaytag</a> - for generating sophisticated HTML tables that includes data paging, grouping, sorting, exporting etc.</li>
</ul>

<p>What other indispensible tag libs are out there?</p>

I use displaytag, and have been quite happy with it. The only other tag library I use is the one that comes with the Stripes Action Framework.
But my truly indispensable Tag library is the one we wrote.
At the office we must have over 50 of them by now for our internal framework.
The key behind them, though, is that they're JSP 2.0 Tag Files rather than class JSP Tags written in Java. Tag files are powerful and very easy to use.
Where classic JSP tag discouraged tag development because it was so difficult, Tag Files encourage it to where it's trivial for refactoring JSP pages now.
I think Tag Files and JSTL turned JSP as a page markup technology from mediocre to one of the best out there.