R is another open source language. Although it is not often employed for MRI processing, it can be useful as a complement for Matlab and Python for statistical analysis in particular.
First, as Python language, R is available as free software, and there are lot of open source libraries available for data analysis. For example, a R.matlab library is designed to read and save R results into Matlab file, which can then be easily extended to Matlab. R language style likes C/C++ and JavaScript, it is easy to use and very popular in statistical computing area.
In addition, RStudio is an integrated development environment (IDE) for R. Just like Matlab command window, rstudio IDE can be used to edit R codes efficiently.
Finally, with the advent of Shiny, it is possible to build interactive web apps straight from R.