Sonnet makes a card for Macintosh, but it's pretty comparable to the ATi Radeon 7000 card (which is a better value/more powerful anyway).
As we all know, you can't just plug a PC video card into a Mac and get it to work. It seems that there is a severe lack of Macintosh programmers out there that know how to program low-level (like the ROMs on the video cards) and are willing to do it -- that's probably the main reason for the lack of Mac video cards.
Another thing is market share -- to make a card cross-platform compatible, you'd have to have a ROM chip twice the size of the current ROM chips to hold both the PC and Mac firmware on the card. That's not cost efficient... which is why we have PC-specific cards and Mac-specific cards. At any rate, you'll notice that most PC cards are based off of one of two chips: either ATi's chip or NVidia's chip. The rest of the cards are just generic knock-offs of those chipsets, and most of those generic companies aren't gonna take a loss in the profits department just to produce a card for a computer that holds only 3% marketshare. That would mean that for every 97 cards they sell for the PC, they sell 3 for the Mac. They'd lose money, because they just produced a whole bunch of Mac cards that don't sell nearly as fast as the PC cards.
