There are several tools around the internet that stimulate game development. They vary from project development, testing tools, versioning and for example texturing. I have been using a few tools for certain development aspects. There exist a lot of tools I am not aware of yet, but I would like to describe 4 tools that helped me with certain game development processes.
If it comes to flexible project planning, Trello is a great tool to organize the workflow for your development team. You can customize different pipelines such as 'to do lists', 'things your team is doing' and 'things that are done'. You can specify these categories to your liking. For example "To do" could also be renamed to anything that suits your companies model. A great thing about Trello is defining 'labels'. You can categorize different expertises by creating a label for this job and assign a 'color' to it. The only downside is, everybody can change "tasks", for example, an audio composer could move a task from an artist to done. I would recommend Trello for 'smaller development teams'. Larger teams require more control and permissioning. Trello is actually a flexible 'scrum' tool. Another great thing about it is that it's hosted online.
Another project planning tool, but this tool is a little more complex compared to Trello. A tool like Jira is best used when you are creating a complex game or product. Jira is better suitable for larger teams. Permissions are easily set-up for project managers, producers and product owners. Jira has a more 'scrumlike' character. Where Trello gives the freedom to define your project flow, Jira has a more standard setting and defines things for you on scrum architecture. It is possible to assign developers on certain tasks and fill in the calculated hours for this. Only people designated to the task or managers can change or move these tasks to different categories. You can say 'Jira' is a more controlled project planning environment.
If you want to test your iOS or Android apps on a larger scale, TestFlight offers a way to spread this easily. You can create an '.ipa' file through Xcode or an '.apk' file for Android and upload this to the TestFlight website. Upload an provisioning profile that lists all devices to grant access to the build you want them to test. Simply update them through mail or through the TestFlight app, and they can start testing your game. You can distribute this around the development team, but you can also invite large groups of players to tests some 'metrics' included in your game. Great for 'quantitative' testing. TestFlight records all 'builds' you have uploaded in the past months, so once people are added to the test team, only an email or notification is required to test a new build.
When it comes to textures I think FilterForge is a great way to improve texture development speed. You can't use these textures for every model in the game, for example characters need a lot more sophisticated textures. However environments, walls, houses, tools, grass, lava or models that can use some speed in texture development. FilterForge uses Photoshop's filters to create unique textures by giving you sliders to set them to your liking. It changes the texture when you just change these settings without having great photoshop filter knowlegde. When your texture is done and you want to have different 'types' such as a 'normal map' or a 'specularmap', FilterForge offers a large scale of different exports of this texture. A tool I have used many times to create great textures for my games.
There exist a lot of tools to stimulate or improve game development, some of them not only applicable on just game development processes. I think every team, company or individual creates a certain 'taste' for what applications or tools they like best. I do want to argue that you shouldn't stop broaden your horizons by knowing or finding new tools. Sometimes they proof to be better than your regular workflow.
Are there any notable tools you use during game development?