Looking for Open Source CRM software.

I am trying to find a solution for an Electric company for Digitizing their work flow. I have used Microsoft's CRM software extensively (don't do it!), and something similar is what I am looking for. I would like to implement an Open Source CRM solution (free preferrably), as the client is rather limited in their funding. Does anyone know of a good Free CRM package using a MySQL database? I am looking for something that is easy to customize, and simple to use. If anyone here has experience with this sort of thing, I would greatly appreciate any information.