BioCro is an R package that was inspired by an earlier mechanistic crop model: WIMOVAC (Windows Intuitive Model of Vegetation response to Atmospheric & Climate Change). WIMOVAC was written in Visual Basic and it is freely available from the link on the left. Details about WIMOVAC can be found in that website. BioCro is mostly written in C and it incorporated optimization routines as well as parameter estimation tools.
I will have the R package here soon as well as some examples and tutorials. I'm also planning to host some free workshops for those interested in using it. Sometime in the future a user interface for the package will also be available.