The latest stable version of amsynth has been released.
For Ubuntu and derivatives there is a ppa.
From the project homepage:
amsynth is a software synth that provides a classic subtractive synthesizer topology, with
Dual oscillators with classic waveforms - sine / saw / square / noise
12/24 dB/oct low/high/band-pass resonant filter
Independent ADSR envelopes for filter & amplitude
LFO which can module the oscillators, filter, and amplitude
Currently it runs as a stand-alone application on Linux, supporting OSS, ALSA and JACK for Audio / MIDI I/O.
It is also available as a DSSI and LV2 plug-in.