Proteus is a popular software tool used for designing and simulating electronic circuits. It's widely used by students, hobbyists, and professionals for circuit design, simulation, and testing.

Proteus offers a free trial version, but it's limited in its functionality and can only be used for a short period. To access the full features, you'll need to purchase a license.

I couldn't find any reliable sources offering a free, valid license key for Proteus. Using a pirated or cracked license key is not recommended, as it may harm your computer or compromise your data.