I'm looking for a well-designed, usable web-based small-business CRM
solution available under an open source license. I prefer minimalistic
design and focus on usability rather than a large feature set.

I would like a comprehensive list of the five or so most popular
web-based small-business open source CRM systems available.
Preferrably including for each product a short description and a
motivation to why it was included in the list.

Attach also if possible a list of candidates that didn't make it to the top five.

Hi,

It is difficult to determine popularity on applications like this as
no one conducts usage surveys, and the applications are generally
intranet only, however I will try to give you a run down of what seem
to be the most mature or complete packages, I hope this is
satisfactory. I am limiting my scope to systems that are based on
proven technologies (PHP, Python, Perl, Ruby, etc) rather than any
custom coded apps, as these may not be nearly as extensible if you
wish to make modifications.

SugarCRM
http://www.sugarcrm.com/crm/
'Commercial Open Source' - SugarCRM is available in an Open Source
edition, but also as commercial products. It does appear to be
legitimate open source, albeit with a unique license rather than an
OSI-approved one.
SugarCRM seems very mature and well-featured.  
Features Include:
- Email
- Lead and Opportunity tracking
- Scheduling
- Task management
- Email campaigns management
- Document storage
- Shared calendar
- Data import options
Online Demo: https://www.sugarcrm.com/crm/demo/sugar-suite.html
Platform: Apache, MySQL, PHP
Licence: Product Specific

Daffodil CRM
Daffodil is a fairly young application, but provides a easy to use and
attractive interface. Available in both free and commercial versions.
Features Include: 
- Lead tracking
- Task management
- Email campaign management
Online Demo: http://www.daffodildb.com:8080/daffodilcrm/userlogin.jsp
Platform: Java
Licence: LGPL

vtiger
vtiger seems to have a very rich feature set. It includes things such
as Inventory management. Also it offers integration with MS Outlook
and Word, and Mozilla Thunderbird via additional open-source add-ons.
Features include:
- Lead tracking
- Email campaign management
- Contact database
- Inventory management
- Knowledge base
- Group calendar
- Customer portal
- Integration with MS Outlook, Word and Mozilla Thunderbird (add-ons)
Online Demo: http://vtiger.com/products/crm/demo_5beta/index.php
Platform: Apache, MySQL, PHP
Licence: Mozilla Public Licence

OpenCRX
http://www.opencrx.org/
Open CRX seems to be a fairly young project, but offers a good range
of features in a seemingly tight package.
- Lead and Opportunity tracking
- Email
- Shared calendar
- Document management
- Task management
Online Demo: http://www.opencrx.org/demo.htm
Platform: Java
Licence: BSD style

These four seem to be the most popular of a wide range. I discovered a
great many CRM applications that are essentially half-finished or seem
to be very industry-specific. The four above represent the most well
developed and widely suitable of the applications I was able to find
and test. They all offer similar core-features around lead tracking
and scheduling.

I am not a regular user of CRM applications, but I am familiar with
the ideas and workflow. However, I have been a web applications
developer. I have tried to make my judgements based on my experience
with web applications and understand of CRM applications.

My pick of the bunch would probably be vtiger, as it seemed to offer
the widest selection of useful features and is also still very
actively developed. Additionally, as a PHP programmer it is what I
personally would feel most comfortable.


Of note:
OFBiz - It isn't a CRM strictly, but provides a basis for additional
software development including at least two CRM applications. I felt
it was a little too cumbersome and sprawling to be seriously
considered.
http://incubator.apache.org/ofbiz/

Compiere - An CRM and ERP application that includes a full POS system.
It is not web-based however. Although it does seem to be in active
development and have a fairly strong following.
http://compiere.org/

Hipergate - Java-based Groupware/Intranet/CRM application - it is full
of features, but a little overloaded and more groupware than CRM.
http://www.hipergate.org/

SourceTap - A promising looking Java-based CRM app. However no relases
appear to have been made since 2004.
http://sourcetapcrm.sourceforge.net/

Centric CRM - Another commercial/free CRM application. It hboats a
wide range of features, but I had problems with the website and was
unable to demo the application.
http://www.centriccrm.com/

In my search I also discovered this brief InfoWorld article from 2005:
http://www.infoworld.com/article/05/08/08/32FEosscrm_1.html?s=feature


Search methods:
I used a combination of Google searches, software portal searches and
social-bookmark searches to narrow the list of applications.

I hope this list is satisfactory. I spent sometime trying to find
another stand-out application to round out my main list to five, but
it really seems that those four are the best options.

Please let me know if I can provide any additional information or
clairification on this answer.

Regards, 
Sycophant