FOSS projects exhibiting clean/good OOP?

[Phillip B Oldham]

I'm wondering whether anyone can offer suggestions on FOSS projects/
apps which exhibit solid OO principles, clean code, good inline
documentation, and sound design principles?

I'm devoting some time to reviewing other people's code to advance my
skills. Its good to review bad code (of which I have more than enough
examples) as well as good, but I'm lacking in finding good examples.

Projects of varying sizes would be great.