Speaking with my follow cph.pm members, tobez and Dimitry a few weeks ago, I asked them about Prima, a Perl-based GUI toolkit they have written. They were a bit sad that Prima was probably not used anywhere. But then today, I saw it mentioned on Perlmonks, so maybe Prima still has a (small?) following.
It made me think about the unthankful task of writing and maintaining an open-source project - unless you screw it up, and get error reports and such from your users, if you actually write something nice and solid, you might never know that people are actually using it in the field.