ATI Technologies Incorporated is a Canadian-based company which specialises in 3D solutions for the home and office market. Founded in 1985, ATI enjoys strong OEM relations - chances are, if you have a Gateway or Compaq, you have an ATI card in your computer. With 3DFX out of the way, Matrox cancelling the G800, and S3 offering only low-end 'value' solutions, ATI is the only real threat to NVIDIA's near-monopoly on the consumer graphics market.
Historically, ATI has a nasty habit of rushing its consumer-end graphics cards to market, and then said cards initially have horrible driver support. Granted, this isn't a problem exclusive to ATI, but it looks like history repeats itself for ATI, leading to a reputation that ATI drivers are extremely buggy. ATI's reputation was further damaged when HardOCP revealed that ATI's Win2K drivers deliberately lowered image quality in order to improve benchmark scores if the drivers detected Quake 3 running. When HardOCP renamed Quake3 to Quack3, ATI suddenly performed slower, but image quality sure looked a whole lot better. To ATI's credit, they released new drivers which fixed this issue.
Nowadays, ATI is neck-and-neck with NVIDIA with ATI's release of their Radeon 8500 chipset. The Radeon 8500 improves on many features found in the original Radeon chipset - Charisma II and Pixel Tapestry II, among others. An interesting feature is TRUFORM. Put briefly, TRUFORM 'rounds' sharp triangles to achieve a more realistic effect with models. It's a bit flawed -- Serious Sam 2's shotgun, for example, looks extremely inflated -- but ATI touts it as a way to improve looks with no impact on performance
Another one of ATI's merits is its All-In-Wonder line. All-In-Wonder is simply a TV tuner, coupled with an excellent hardware DVD coder, stuck onto whatever ATI's top technology is at the moment; right now it's the Radeon 8500. ATI has vastly improved their All-In-Wonder line over previous versions; if you're looking for a combo video card and TV capture card, this card may be the one to buy.
Recently, ATI has shown great improvement. A test of the up-and-coming Unreal II engine shows ATI's Radeon 8500 topping the GeForce in nearly every benchmark test. Of course the GeForce4 wasn't tested....