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.