<p>What libraries can I use to build a GUI for an Erlang application? Please one option per answer.</p>

Most people don't code the actual GUI in Erlang.  A more common approach would be to write the GUI layer in Java or C# and then talk to your Erlang app via a socket or pipe.
With that in mind, you probably want to look into various libraries for doing RPC between java or .Net applications and Erlang:
http://weblogs.asp.net/nleghari/archive/2008/01/08/integrating-net-and-erlang-using-otp-net.aspx
http://www.theserverside.com/tt/articles/article.tss?l=IntegratingJavaandErlang
EDIT
If you're truly set on coding an interface in erlang, you might consider doing a web-based GUI served via Yaws, the erlang web server:
http://yaws.hyber.org/appmods.yaws