Mantis is an issue tracker that is implemented in PHP. The main features include:

• Free (GPL License)
• Easy to Install (both internally and in hosted environments).
• Easy to evaluate
  • Demo
  • InstantMantis - Be up and running in 2 minutes (not recommended for production use)
  • Mantis2Go - Try MantisBT on Windows (not for production use)
• Simple User Experience
• Web Based
• Supports any platform that runs PHP (Windows, Linux, Mac, Solaris, AS400/i5, etc)
• Available in 68 localizations.
• Customizable Issue Pages
• Multiple Projects per instance
• Support for Projects, Sub-Projects, and Categories.
• Users can have a different access level per project
• Changelog Support
• Roadmap
• My View Page
• Search and Filter
  • Full Text Search
  • Simple/Advanced Filters
  • Shared Filters (across users / projects)
• Built-in Reporting (reports / graphs)
• Custom Fields
• Email notifications
• Users can monitor specific issues
• Attachments (can be saved on webserver or in database - can also backup to an FTP account)
• Issue Change History
• RSS Feeds (news, issues matching saved filters, issues matching a specific project)
• Customizable issue workflow
• Sponsorships Support - users are able to place bounties or sponsorships for specific issues, also developers can track such sponsorships / payments.
• Anonymous Access
• Signup with Captcha and Email Confirmation.
• Self-Service Reset Password Support
• Export to csv, Microsoft Excel, Microsoft Word
• Ability to extended functionality through hook (custom) functions.
• Ability to extended functionality through plugins.
• Reporting issues via Email (available as a patch for 1.1.x and EmailReporting Plugin for 1.2.x]])
• Reporting Issues via a custom form on your website (Anonymantis).
• Source Control Integration (GIT, SVN and HG).
• No limit on the number of users, issues, or projects.
• Wiki Integration (optional)
• Time Tracking
• Ability to send messages to messages to other users in regards to a specific issue.
• Public / Private Projects - Public project accessible to all users, private are only accessible to those explicitly added.
• Public / Private Notes - Private notes are accessible to users with a specific access level to the relevant project.
• Public / Private Issues - Private issues are accessible to users with a specific access level to the relevant project.
• Issue Relationships
• Issue Relationship Graphs (uses “dot” library).
• Attachment Auto-Preview
• Public / Private News (news can be associated with a specific project, or with all projects).
• Sticky Issues (always appear on the top of the issues list).
• Group Actions: actions can be applied on multiple issues.
• Easy hyperlinks to issues and issue notes (e.g. #123 hyper links to issue number 123).
• Ability to view recently visited issues (the last 5 visited issues are visible by default at the top right corner).
• Authentication
  • Default Mantis Authentication (recommended)
  • LDAP Integration
  • HTTP Basic Authentication Support
  • Active Directory Integration (patches available)
• Chat Integration (optional)
• Multi-DBMS Support - Mantis uses ADODB as an abstraction library to support multiple DBMSes.
  • MySQL
  • MS SQL
  • PostgreSQL
  • Oracle (experimental)
  • DB2 (in progress)
• Webservice (SOAP) interface (MantisConnect)
  • A SOAP web service that is implemented in PHP and can be consumed from any language that supports SOAP web services.
  • .NET Client Library
  • Java Client Library
  • Cocoa Client Library
  • Eclipse Plugin
  • NAnt Task
  • Mantis Notifier
• Support for mobile devices (MantisWAP).
• There are community projects that integrate it with content management and project management too
• Twitter Integration allowing users to monitor updates (Added in release 1.1.0a4)