Let me stress that the most important things about design tools:
- They must be something that you are comfortable using. (Otherwise, you just won't!)
- It must be something that allows you to easily share with your team. (Otherwise, you just won't!)
My #1 tool for design is Google Drive. Nothing beats Drive for ease of sharing, ease of use, number of tools and price. Plus, I have plenty of backup!
Sharing on Google Drive is about as easy as it can be. I create as many documents as I feel like creating. Then I can put them all in a folder and share them in total, or I can share them one at a time. I can allow people to only View my documents, allow them to View and Comment, or give them total access to my files. Removing access from people occurs in two clicks.
Ease of Use
The spreadsheet, word processor, and draw programs work pretty much like any other ones. They're extremely intuitive and easy to learn, and in addition, they're quite powerful. I can access all my files from any computer with internet access as well. (I normally access my account only with my own computers, though, since security is important to us.)
Number of Tools
Drive allows you to create all the basic files you need for game design (or any design, really): spreadsheets, presentations, documents, forms, and an endless number of apps. (It's not really endless, but I haven't been able to go through them all yet.)
Simple, it's free. I love it!
Currently, I'm exploring a few more tools for game design as I'm always looking for better ways to communicate with our team and keep organized. I may have found it in the Gingko App, but I won't know until I've had time to sit down and really give their trial a workout.
In addition to Drive, I use Photoshop and Paint to create physical representations of design systems. There's several free equivalents to these programs. I also use physical white boards to help. In fact, we painted one of our walls to be a clear whiteboard. It gets a good workout during our team meetings.