Currently I am a heavy contributor to the TYPOlight CMS. http://www.typolight.org
My efforts include (but are not limited to) integrating all different kinds of APIs and tracking bugs in the core.
Therefore my skills are PHP in there.
I also was (and still am but pretty much retired in the past time) a developer for JEDIVCS, a version control system written in Borland Delphi for Project JEDI.
Aside from that I dig through all variants of OSS projects I use and provide patches and bug reports for stuff I find.