I am not sure if you are after a free CRM system (I know you said open source and free), or are willing to pay - I will base my opinion off using both free and paid. You also did not really state the nature of your customers, so this would change my overall answer. I would suggest perhaps trying
Really Simple Systems which you can try from this website. (Please be aware I trialled this program at one point for my own personal use, but will not make any references from this.)
If you look into their price comparison section, I think the following is of use:
For free use, you get 100 "sales accounts", with flexible payment options. Note that a sales account is defined as:
An Account is a company or organisation that you deal with, including your customers and prospects
Unlimited contacts, which are defined as:
Contacts are the people who work in your Accounts
Availability to add custom fields to a customer record, defined as:
Add new fields and change existing fields to Accounts, Contacts, Campaigns etc
Cross platform support on Tablet and Phone, as:
Works seamlessly on smartphones and tablets
Limited Outlook intergration, with the following restrictions:
Sync your Outlook Calendar with Tasks in the CRM.
Works with Outlook365 and hosted Exchange only.
Lastly, Google Integration:
Sync the Contacts in your CRM with Google Contacts, and then to your smartphone.
Utilizing the custom fields, allows you to run your own custom reports (supported with the free versions) and store a limited 100MB of data.
Personally, I had no issues using the free version, and it was backed with full support. I will attach some other reviews of the product from third party websites to allow you to get a full grasp:
What does this cover?
- Stores Contact Data (to a degree)
What doesn't this cover?
- Possible multi-user access
- Latest update list
- Open Source
I hope this helps you.