WhySynth is a versatile softsynth which operates as a plugin for the
DSSI Soft Synth Interface and is useable with apps like Qtractor and
Ardour.  It features 4 oscillators, 2 filters, 3 LFOs, and 5 env
generators per voice, plus effects.

It also has many pre-sets, so if you just want to install a DAW and
a soft synth so you can start noodling around with neat synth sounds,
this is a good one to start with.

You'll probably also want to install a DAW like Qtractor or Ardour.