At Undo, the company where I work, we have a quite big C codebase with a not very consistent style.
To improve things, we decided to use the clang-format tool (part of the LLVM projects) to enforce a consistent style for new and refactored code.
We don’t want to change all the existing code to avoid a massive and confusing change, and we don’t want spurious unrelated changes when somebody modifies a file.

To achieve this, I wrote a couple of scripts which, using clang-format and clang-format-diff, only modify the formatting of the code you are about to commit.

The most interesting part is, I think, the pre-commit hook which suggests fixes before your code is committed:

This code is now available in the clang-format-hooks repository on GitHub.