Open Source – Web Office: OpenGoo
It's PHP & MySQL and so would be easy to deploy on any environment while for Zimbra you have to use RedHat or CentOS as a platform. It is very feature-rich with lots of feature listed below here from the official website FengOffice.
The Overview shows a summary of management information. It displays relevant information like: schedule of coming and current activities, running tasks, overdue and pending tasks, recently added or modified documents, latest messages, comments and conversations, e-mails, and more.
Workspaces allow individuals and teams to organize their data and tasks, and share them with other people. They can be linked to Clients, Projects, Products, Services, Departments, etc. You can define many levels and sub-levels of Workspaces. Visually identify your most relevant Workspaces, quickly, by assigning different color labels.
Notes are an easy way to register and share comments over any piece of information. They are very useful for: tracking the conversations maintained with third parties, assessing the client satisfaction about quality, tracking a physical document, etc. A note could be entered by any user, and can notify others about it by e-mail.
The e-mail follows the traditional scheme with one or more addressees who exchange messages. Additionally, users have the possibility of sharing the message with others by sending the message to a shared workspace. E-mail attachments are automatically stored as Documents.
The contacts module manages the information of all organizations and people linked to your company. The contact list can be filtered by a specific workspace, by tags, or in a general manner.
The calendar allows scheduling an agenda of events, milestones, tasks with deadlines, and meetings. It can be displayed daily, weekly or monthly. The schedule could be shared amongst the users who have the correspondent privileges.
You can see the schedule of events in general or specifically for each workspace or tag.
Feng Office allows you to store and share all types of documents. The system keeps record of the different versions, comments, and a controlled access to optimize the work of people who collaborate on a document. Text documents and presentations can be created or edited directly in Feng Office without the requirement of any additional software in your computer.
You can track every task, sub-task and milestone that is part of a service, project or activity. The system provides quick access to the tasks, grouped by state (pending, behind schedule and completed), priority, assigned person, and more. Tasks can be grouped using milestones, setting common deadlines and improving project management.
It is possible to notify a person who is assigned a task by e-mail. All the information linked to a specific task is available at a single click.
You can create a template out of a set of tasks and sub-tasks, improving the adoption of standard procedures.
The system allows the tracking of hours for each workspace, sub-space, task and user. Reporting capabilities allow to easily produce a list of hours for any time period, users and workspace.
Web-links are hyperlinks to external Websites. They can be classified by tags, linked to other information and assigned to any number of workspaces.
SEARCH & FILTERS
The information can be quickly accessed using the integrated search engine, or it can be found using the filters of type, tags or workspaces.
You can modify, register or create new users through the user panel. You can create and modify groups of users and group or individual permissions, making available only the relevant and corresponding information to each user.
All the information managed by the system can be linked, assuring no element is forgotten or goes unnoticed. One document can have messages linked to it -or a meeting can have documents- and so with every element of the system, thus creating infinite data management possibilities.
From all of them I really like Tasks (which has got milestones and sub-tasks), Calendar and Custom Reports. Web-links and other features could be still improved. Hopefully we will see lot of improvements in future releases.