FireBreath
Home page: http://www.firebreath.org
FireBreath is a cross-platform browser plugin framework. It supports ActiveX and NPAPI on Windows, Mac OS X, and Linux (probably other unix-derivitives as well).
Getting the source
After checking out the source, make sure you run:
git submodule update --init --recursive
in the project root to get the linked repositories. The main one you want is the firebreath-boost repository. You don't need firebreath-boost if you use WITH_SYSTEM_BOOST.
Using Subversion
If you choose to use subversion to check out FireBreath, make sure you also check out the firebreath-boost project as src/3rdParty/boost/ (you'll have to remove the existing directory).
Good luck!