What open source(or not) CRM do you use ? i have come across Tactilecrm and it's kinda' great.
I've worked with Sugar some. It's pretty decent, though last I saw their internal architecture was kind of a mess, and some sort of initiative to use Smarty wherever possible had made it worse. (I was less concerned with using it, more concerned with extending it.)
my workplace currently uses vtiger
I've been using FatFreeCRM, it provides the basic and is then relatively easy to add features using Rails plugins.
drupal is very lightweight and I found it to be rather easy to use and develop for