I have used many modeling and simulation tools. I even worked for a tool vendor once, 15 years ago: Powersim. But lately my favorite tool is Forio Simulate. Why Simulate? Three reasons.

First, Simulate is a system dynamics tool. As with all system dynamics tools, you can use it to model causal loops, where A causes B which causes C which causes A. For example:

1. Greater market share of an operating system causes that operating system to be more attractive to application developers, as they perceive a bigger market for their wares.
2. An operating system that is more attractive to application developers leads to more applications written for that operating system.
3. An operating system with more applications is more attractive to customers.
4. An operating system that is more attractive to customers leads to even greater market share.

Causal loops are ubiquitous in any system in which people participate.

Second, Simulate makes finished simulations easy to distribute to users. To a user, a simulation is just a web application: point your favorite browser at the right URL, login, and go. No installation required.

Third, Simulate supports attractive user interfaces. I build models with user interfaces, so users can experiment with the models, even change the underlying assumptions. Simulate supports user interfaces, but more importantly, it supports attractive user interfaces. Aesthetics is important, because people evaluate models based on how they look. There is even some evidence that attractive user interfaces are easier to use.

Do I use other tools? Of course. I often start a project by working in Excel, and some projects never leave the spreadsheet. I have worked in several business process modeling tools. I have done some recent work in R, and I am excited about shiny, a new tool for distributing R models as web applications. But Simulate is my go-to tool.