There's nothing that works *perfect*.
There are basically 3 choices... ATI, NVidia, and Intel.
Intel is the lower rung in terms of performance. They generally have open sourced drivers that work well, however, there can sometimes be issues with particular resolutions requiring hacks. Unfortunately, I don't think they make *cards*, their graphics chips tend to built into mainboards/laptops.
NVidia will probably get the most votes. They tend to be good performing and have had a history of better drivers. Lately though, their drivers have regressed very significantly.
ATI has historically had very bad Linux drivers, however since the acquisition by AMD, their drivers have improved DRAMATICALLY. I would rate them as equal (if not superior) to the Nvidia drivers, and they're only improving. Additionally, they have shown a commitment to open sourced drivers and are helping in the development of such drivers. They can't just give out the source for their closed source drivers since there are licensed components that they don't have the right to give out.
So with that info, you can pretty much take your pick of any ATI or NVidia card that supports dual monitors.