3dfx's Voodoo cards (at least the Voodoo 3 and 4) were shipped as flashable to either platform. The drivers and utilities were once available on 3dfx's website. I doubt nVidia kept any of them around, though. I'm using a Voodoo3 in one of my G3s, flashed over from PC, and it was no hassle and works great.
The Voodoo5 is a bit different, I think. I have the Mac version. I've never seen the PC version, but apparently only the Mac version has DVI plus VGA, the PC model has dual VGA.
AFAIK, no nVidia card will work in a Mac prior to the GeForce2, simply because there were no Mac products available prior to that. Some PC GeForce2 or better cards will work in Macs, but some won't. It depends.
Several ATI cards, such as those with Mach 64 and Rage 128-based chipsets, were shipped with Macs, and some similar PC models are flashable. The Rage 128 models are pretty straight-forward, actually. Other ATI products, though they were used in Macs (such as the RADEON 7500, etc), aren't flashable from PC. Some are, but most aren't.
I don't think Apple ever used any S3 or Trident graphics solutions, nor do I recall any being available for Mac, so I wouldn't even try with those. Other brands Apple used, such as Chips & Technologies and Western Digital, were typically only used in PowerBooks, as their products were usually specifically designed to drive LCDs.