Qt, LibreOffice, OpenStack, Linux itself, WebKit, GStreamer, Wayland just to name a few have both paid and freetime contributions (often from the same people too).

Neil works for Collabora, we contribute to quite a few projects in our paid time. ;)

Numerous Apache projects are developed primarily by paid contributors.

I know that one of the libsass developers is paid to work on it.

GNU MediaGoblin, MusicBrainz