Nearly all Hams know DRM (Digital Radio Mondial) as the new Digital shortwave radio standard. There are already some decoders like the commercial Fraunhofer on http://www.drmrx.org/ or the open source decoder [DReaM] by Volker Fischer on http://www.sourceforge.net/projects/drm
The latest CVS build of DReaM just included an experimental transmitter mode allowing to send Slide Show images but no digital voice is possible as there's no free DRM compatible AAC encoder (AAC is the voice compression used)
Now HB9TLK had the idea of substituting the AAC compression part of DReaM with a LPC-10 compressor from Hawksoft.
He included the hawkvoice library and modified DReaM a little bit and now it is possible to transmit digital voice on HF without even modifying your transceiver - the signal has a bandwidth of 2 kHz - fitting into a normal SSB "channel"
Of course this is not a DRM compatible mode (too small bndwidth and LPC instead of AAC compression) but it seems to be the first public available software allowing digital voice QSOs on HF with a standard radio and a Windows/Linux PC only.

And the best: it's completely free as all parts are released under the terms of the GPL licence. And it is available instantly - instead of the often announced SKYWAVE 3K which seems to be vaporware.

You'll find this sofware on HB9TLK's website including all sources for building your own version.

As qsl.net seems to be very slow at the moment I mirrored the file on my server. Direct download