I think having a self-explanatory link that makes people want to
register is the best. OpenACS has done a good job, unlike ArsDigita
where you have to "become a partner" to register your site.
What I want to see is a more regionally divided list of users. Much to
my own surprise I recently made another ACS hacker here in Berlin,
Germany. And meeting people f2f is still the best.
That said: I will be at this even: http://www.mdlug.de/linuxtag.php3
and talk about the ACS.