This is a list of tools that usually cost money but are free for open source projects. If you know of any more, please let us know.
AppCode – A new Objective-C IDE based on IntelliJ platform, for developers building apps for Apple devices such as Macs, iPhones & iPads.
IntelliJ IDEA – IntelliJ IDEA is recognized by many Java developers and industry experts as the best Java IDE on the market.
PhpStorm – Professional IDE (Integrated Development Environment) for Web and PHP developers.
PyCharm – Lightweight IDE for Python programming language with support of Web development with Django framework and Google App Engine.
RubyMine – Ruby and Rails IDE providing the most essential tools for developers, all integrated together to ensure a convenient environment for Web development with Ruby on Rails.
Wing IDE – Wingware's Python IDE is an Integrated Development Environment designed specifically for the Python programming language.
Development Environment Extensions
Source Control Systems
Assembla – Ticketing and issue management, Subversion, Git, Mercurial, Wiki, and other collaboration tools to accelerate development.
bitbucket – Unlimited private code hosting for Git and Mercurial.
GitHub – GitHub is a web-based hosting service for software development projects that use the Git revision control system.
Bug Reporting and Issue Tracking Systems
JIRA – Track everything – bugs, tasks, projects, people, source – and pull reports for easy analysis and management.
YouTrack – YouTrack is a keyboard-centric bug and issue tracker web application that saves your time spent on issue processing.
Confluence – Content collaboration tool for teams to create, share, and discuss rich content – projects, documentation, files, ideas, minutes, specs, diagrams, mockups, anything.
Project Management Tools
Bamboo – Continuous integration, continuous deployment and release management.
Crucible – Find bugs and improve code quality through peer feedback.
FishEye – Browse, search, and manage your source code repositories.
GreenHopper – GreenHopper adds agile project management to any JIRA project.
Pivotal Tracker – Pivotal Tracker is an easy to use, agile project management tool that brings focused collaboration to software development teams.
TeamCity – TeamCity is a continuous integration & build management solution which, following the tradition of IntelliJ IDEA, takes software team productivity to a whole new level.
Translation and Localization Tools
Amanuens – Amanuens is a software localization platform based on the Microsoft Windows Azure Platform.
Crowdin – Crowdin is a complete solution to make your website or software universally accessible through translation.
Get Localization – Get Localization brings Developers and Translators together to create applications everyone can understand.
Testing and Performance Tools
Bonfire – Rapid bug reporting for session-based web application testing.
Clover – Prioritize time spent creating and updating tests, and speed your builds.
Coverity Scan – Thousands of open source developers rely on Coverity to help them enforce the quality, safety, and security of open source software.
dotCover – An extremely simple to use .NET code coverage tool that analyzes statement-level coverage in .NET Framework 1.0-4.0 and Silverlight applications. dotCover integrates with both Visual Studio and ReSharper.
dotTrace Memory – A complete profiling solution for detecting memory leaks in .NET applications.
dotTrace Performance – Powerful profiler that helps .NET developers investigate performance problems, spot and remove performance bottlenecks in applications based on .NET Framework 1.0-4.0, Windows Mobile, and Silverlight 4.
Gun.io – Gun.io is a place for talented hackers to hire each other for small tasks.
Spoon Studio – Run your applications with no installs, dependencies, or conflicts. Deploy in standalone EXEs, with Spoon Server, or on Spoon.net.
Tender Support – A simple helpdesk application that you and your users will love.