Ardour has been updated to v5.5, with a variety of new features and many notable and not-so-notable fixes.
Among the notable new features are support for VST 2.4 plugins on OS X, the ability to have MIDI input follow MIDI track selection, support for Steinberg CC121, Avid Artist & Artist Mix Control surfaces, "fanning out" of instrument outputs to new tracks/busses and the often requested ability to do horizontal zoom via vertical dragging on the rulers. There are also the usual always-ongoing improvements to scripting and OSC support.
As in the past, some features including OS X VST support, Instrument Fanout, and Avid Artist support were made possible by sponsorship from Harrison Consoles.
More details at ardour.org/whatsnew.html