The ADT Plug-In takes a mono input signal and creates a stereo output. The original input signal will be output on one channel, the new ADT signal on the other. Blending the two is also possible.My opinion ? I use only few freeware plugin effects - mostly I just use Ableton internal plugins. But thanks to ADT plugin I don't need to clone and pan / delay channels for stereo effect. It's very easy to use and works great as preview (and of course as final effect too). It's definitely one of my favourite freeware plugins - I use it almost in every project - works both for vocals and instruments. ADT just improves my workflow nicely.
ps. There's also AU version for Mac users.
Download: ADT - Artificial Double Tracking