Beware, long intro. Like a good Pink Floyd song. When browsing the http://homerecording.be forum I've seen two nice gems inside the "downloads" section: autotalent and a Yamaha C7 grand piano.
I have been wary of relying too much on external plugins. Switching from Cubase to Ableton Live, from Windows to Mac and from PowerPC to Intel processor, from Leopard to Snow Leopard, have all caused some of my plugins to not work anymore. Commonly, free plugins are also "abandonware" and are not updated to newer platforms. As a result, you load your song and find some FX or instruments missing.
As a result, I've chosen to stay within Live as much as possible: use native plugins and instruments. The 'bare-bones' version of Live comes with few instruments, but they are versatile to do enough. And I've added operator, which is such an incredibly flexible sound beast, that it goes from smooth to edgy, from musical to rhythmical.
Autotalent
Autotalent is a real-time pitch correction plugin. It is free, Open Source and cross-platform. In fact, it exists in its original form as a LADSPA version for Linux made by Tom Baran (http://web.mit.edu/tbaran/www/autotalent.html) and was ported to VST and AU by Oli Larkin (http://www.olilarkin.co.uk/index.php?p=freeplugins#at).
I've tried it inside Ableton Live on Mac and it works fine. You can use it on monophonic input (e.g. your voice) and enforce the notes to be only certain pitches. If you overdo it, you get artificial effects, but as proven by the giant hit "Believe" by Cher, this is not always a problem if you can original.
I also found a nice effect: set the LFO filter to 1.0, frequency to 10Hz and use a square wave. The result? You get both a low and a high-pitched voice from a single phrase.
(read on http://www.homerecording.be/download/1450)
Sonart Yamaha C7 Grand Piano
The second gem was a free Yamaha C7 grand piano, accurately sampled by Sonart (http://sonart.cc/shop/product_info.php?cPath=62&products_id=210). It is available in multiple formats, but contains a rather large sample library.