GF2 and older have almost useless anti-aliasing and anisotropic capabilities. Extremely inefficient and ineffectual ordered grid supersampling (SSAA). Minimal anisotropic support.
GF3-4 have multisampling AA (MSAA is edge only) and Quincunx post process mode. The MSAA is ordered grid which is not very effective but it is at least usable because it is more efficient. Quincunx is a post process filter which can be interesting at high resolutions. Anisotropic is much improved but very slow. Max 4x SSAA.
GF5-7 have similar anisotropic and anti-aliasing capabilities (and limitations). Rotated grid MSAA is decent but it is inferior to ATI gamma corrected rotated grid MSAA. Anisotropic has been optimized and is quite usable. Many unofficial AA modes like Quincunx, ordered grid SSAA, hybrid MSAA+SSAA and Gaussian are available via Rivatuner. Max 4x MSAA.
Radeon is not very useful for the games people here want to play. They are best for D3D7+. Forget non Quake OpenGL games and D3D5. Still, to be thorough :
R100 has ordered grid SSAA like GF2 and it is equally weak. However it has a speed optimized anisotropic that while having implementation caveats is much better than nothing.
R200 is very similar. It supposedly has SSAA with a programmable sample pattern but practically it is again ordered grid SSAA. Anisotropic is also similar. GF3 anisotropic is higher quality but since you can't practically use it so what.
R300+ have superb gamma corrected rotated grid MSAA and top notch anisotropic capabilities.Max 6x MSAA. NVIDIA didn't catch up until GF8.
For some reason ATi refused to implement SSAA again until the 5000 series.
Voodoo5 has fantastic rotated grid SSAA but of course this is extremely inefficient. 4x SSAA essentially requires 640x480 to be playable but it is wonderful for old Glide games. Radeon 5000+ officially support a similar mode. Voodoo5 does not do anisotropic. Some people tweak texture LOD to reduce mipmap blurring with SSAA.