While I joined to get turned on to new (and old) stuff I havn't heard yet, I could suggest a few too. I'm a bit surprised by the minimal Frank Zappa selections. One size fits all, Apostrophe, Overnight Sensation and Zoot Alures would be excellent sources for proggish Frank.
The other band that quickly comes to mind is Crack the Sky. Any and all. Great stuff !
And so since I'm new here I'll sit back and enjoy all the great stuff that's already here... being sure not to request all the stuff I already have on CD.