1. Cost: The main difference between Redmine and Zammad lies in the cost aspect. Redmine is an open-source project management tool that is available for free, which makes it a suitable choice for small businesses and individuals with limited budgets. On the other hand, Zammad is a commercial help desk and ticketing system that requires a subscription fee. This makes Zammad a more suitable option for larger organizations with specific support requirements.

2. Features: Redmine offers a wide range of features that make it a comprehensive project management tool. It includes capabilities such as issue tracking, time tracking, wiki, forums, Gantt charts, and more. On the other hand, Zammad focuses more on support and ticket management features. It provides functionalities like ticket creation, automatic ticket distribution, knowledge base, and customer communication channels. Therefore, the choice between Redmine and Zammad depends on the specific needs of the organization, whether it requires a project management tool or primarily a help desk system.

3. User Interface: Redmine has a somewhat complex user interface compared to Zammad. Redmine's interface can be overwhelming for new users, as it includes a lot of options and settings. Zammad, on the other hand, has a more user-friendly and intuitive interface, making it easier for users to navigate and perform tasks. This can be beneficial for organizations with a large number of non-technical users who may find it easier to use Zammad.

4. Customization: Redmine offers a high level of customization, allowing users to tailor the tool to their specific needs. Users can create custom fields, modify workflows, and adapt the tool to fit their unique processes. Zammad, on the other hand, has limited customization options. Users can make basic configuration changes but may not have the same level of flexibility as Redmine. Therefore, organizations that require extensive customization may prefer Redmine over Zammad.

5. Integration: Redmine has a wide range of plugins and integrations available, allowing users to extend its functionality and integrate it with other tools. This flexibility enables seamless collaboration and data exchange between different systems. Zammad, on the other hand, has a smaller number of integrations available, which may limit its ability to connect with other tools commonly used in organizations. Therefore, the availability of integrations should be considered when choosing between Redmine and Zammad.

6. Support and Documentation: Due to its open-source nature, Redmine has a large and active community that provides support and documentation resources. Users can find forums, guides, and tutorials to help them troubleshoot issues or learn how to use specific features. Zammad, being a commercial tool, offers direct support to its customers. This means that users can reach out to the Zammad support team for assistance. The availability of community support versus direct support can be an important factor for organizations when selecting between Redmine and Zammad.

In summary, the key differences between Redmine and Zammad can be categorized based on cost, features, user interface, customization options, integration possibilities, and support/documentation availability. These factors should be considered in relation to the organization's specific requirements and preferences to choose the most suitable tool.