I have experience in C/C++ and C#, and I'm planning to create a large software with the help of my friends, which will also include a GUI. It will focus on computer security and analysis, something like penetration testing, risk management, etc. The point is that I want it to be multi-platform software. I want it especially to work in Windows, Linux and Mac OS X. The point is that I don't know which programming language and programming paradigms/tools to use for this purpose. I want to hear your suggestions, any suggestion is highly welcomed. I plan to do several large software developments in the next few years, therefore, I'm even highly willing to learn new programming languages or tools, which will be useful and ease my job.