The Insight Toolkit (aka ITK) provides a well documented, superbly constructed image processing foundation that is used as the basis for much of the data analysis done in our lab.
The wealth of algorithms available has greatly accelerated our study of brain disease by allowing our group to try new methods quickly in a prototype environment, and then rapidly move them into fullly tested production mode.
In addition to the great image processing methods, the ITK toolkit provides many examples of how to construct maintainable tools from a software engineering perspective. Recommendations on coding style, application/algorithm decoupling, code testing, code coverage, and code dynamic analysis examples are a great way to ensure that tools built on top of ITK survive the beyond the original developers interest in the software.