- Both sketch and invision are easy to use and quick to setup. You can create prototypes for different device types. There are many plugins available that allow you (or your designers) to work even faster. For bootstrapping the one month free trial of sketch and the one prototype for free version of invision offer a great option to build a prototype without any cost.
- Atomic is one of my favorite tools. It's quite flexible and will work for low- and high-fidelity prototyping. It's simple to get started as a non-designer, as Atomic recently released a series of videos. You can collaborate with others on your team in real-time, so you can build prototypes quickly. Highly recommend Atomic!