Useful links concerning opensource Radeon support in Linux

RIV@NVX

Radeon HD driver - supports R500 and R600 series, including IGP and mobility
RadeonHD.org | IRC Archives For The Radeon & RadeonHD Drivers - Radeon HD driver news, hosted by Phoronix, has IRC logs of both #radeon and #radeonhd channels, very useful
X.Org Wiki - radeonhd - Radeon HD driver basic info and download location
freedesktop.org git - xorg/driver/xf86-video-radeonhd - git web view of radeonhd repository, useful for tracking lastest changes

Radeon driver - supports all Radeons, including IGP and mobility versions
X.Org Wiki - radeon - Radeon driver basic info and download location
DRI Wiki - ATIRadeon - DRI status on radeons, a lot of information about actual cards
freedesktop.org git - xorg/driver/xf86-video-ati - git web view for radeon driver

RIV@NVX

To add some more information, let's try to answer the often asked question:
"Which driver is most appropriate for me?"

First off, there are three drivers for Radeon cards in Linux:

1. opensource radeon driver (supports all radeons)
2. opensource radeonhd driver (suppports R500 and up, including RS600 and RS690)
3. proprietary fglrx driver (supports R300 and up)

There used to be reverse-engineered avivo driver, but it isn't developed anymore.

If you don't need 3D, situation is rather simple...

• If you have R100/R200/R300/R400, use radeon driver. Most distributions ship it by default, so there is no need to change anything at all.
• If you have R500/R600/R700, you can choose between radeon and radeonhd driver. Both will work, but one might work better at resolution detection or some specific feature you care about (for example, video support). Regardless, any but reports are welcome. So, if you find out that one driver does something, and other doesn't work that well, please report it - it will help make it work better in the future.

On the other hand, if you need 3D, it is a bit more complicated...

• If you have R100/R200/R300/R400, it is recommended to use opensource radeon driver. You will get great 2D and 3D. It might not be as fast as it would be with fglrx driver (on R300/R400), but it will work stable.
• If you have R500, RS600 or RS690, you can use recent opensource stuff (Mesa git with radeon or radeonhd git) and get 3D (in case of radeonhd, enabling 3D acceleration disables 2D acceleration; this will be fixed in the future). It works, but it's not yet very polished.
  Also, IT IS MUCH SLOWER THAN fglrx most of the time. If you want it to work faster, use fglrx driver. This will be improved in the future, most likely before the end of the year when memory manager (TTM or GEM) starts working.
• If you have R600, RS780 or R700, you will need to use flgrx to get 3D. There is no opensource 3D support on these chips, but it's in the works. It is expected before the end of the year.

Hope this helps