Kernel 2.6.38 introduced an API to access the kernel crypto API from userspace. While there was a port of BSD's cryptodev for linux which basically provides the same functionality, the cryptodev code never made it into the mainline of the kernel.
Accessing the kernels crypto API from userspace allows making use of crypto hardware, which can't be accessed from userspace directly. Hardware accelerated cryptography as provided by VIA Padlock1) and Intel AES-NI2) can be accessed from userspace directly, so you do not need AF_ALG at all, but AMD Geode processors AES cryptography is - contrary to Padlock and AES-NI - not an instruction3) and therefore can't be accessed from userspace.
You may be interested in the discussion of af_alg vs cryptodev performance 4) and the raw numbers and benchmarking software too
As I own AMD Geode powered hardware (ALIX), I decided to play with AF_ALG.