We've added Bitbucket as a software version control repository, since it is free for small teams and has the same functionality as GitHub.
InVision lets you create interactive mockups out of static screens. You upload images of your website or app (can even be scans and sketches) and then map out where people click and what screen it takes you to. Gives a good way to demo an app concept or test how people interact with your design.
This and Asana are good project management tools if you use them effectively. If the whole team is signed on and actively engaging with it, it’s a good way to track progress for a lot of different tasks at once and make sure nothing gets forgotten. However, if people don’t engage with it, it can seem more like a pointless exercise than a useful management tool.