Dark is a free VST / AU for creating and playing rich, dark pads. There are eight rectangles at the top of the instrument, one of which is grey (this one is un-muted). Each one is a trigger for a sample. The tone changes from grey to varying lighter tones of grey to let you know which ones are in use.
Palette is a VST/AU plugin for creating and playing rich, evolving pads on Mac and Windows platforms.
Tycho is free a spatial, cosmic, sci-fi oriented VST synth instrument, the emphasys is on expresiveness: velocity and aftertouch (no polyphonic aftertouch, alas) can do a lot to the sound, so try it with a full featured MIDI keyboard.
Baby Bion is a free VST synth that specializes in atmospheric sound due to the built in Soundscaper, uses Bion’s oscillator.
Regen is at its heart a looper plugin that is well suited for ambient / textural / soundscape type layering. It can be a simple looper but also offers deeper dive controls to modify playback of recorded material.