I don't think there is any simple CRM software; I have managed all my small business needs for over twenty years just by using first dBASE and then mysql tables.

There is a comparison of CRM software at http://en.wikipedia.org/wiki/Comparison_of_CRM_systems which may be a better starting point if you want something more substantial than database tables.

moved to office and productivity, does not belong in Calligra

The following are all open source solutions:

XRMS CRM
vtiger CRM
SugarCRM
info@hand