The following items are common accessories for the AK.  Most of these are legal to own in the US, however some are banned from legal ownership.  High capacity magazines are now banned from import, though you may still purchase, own, and use whatever is already in the country.  Talk to your local gunsmith or deal for more information.

Bayonet (various)

Flash Suppressor / Muzzle Brake (various)
               

Recoil Buffer (Buffer Technologies)

Magazines: 5 round, 30 round, 40 round, 100 round (drum)
              

The most common AK magazine is the 7.62x39 Chinese (Norinco) steel version.  However, Germany and other European countries also manufacture them.  You can tell the difference between a Chinese and European magazine by the presence of a steel "rib" along the length of the back of the European ones - this makes them slightly more durable.  40 round magazines are special treats, but don't take them to a public range, as they tend to scare a lot of people.  Magazines for the 5.45x39 AK rifles are harder to come by in the US, but they are here.  You will generally only be able to find 5 and 30 round versions of the 5.45.  Any AK magazine will fit any AK rifle, as long as the chambering is the same (don't expect a 5.45 magazines to click into a 7.62 AK rifle.

Another feeding option is the drum magazine, which is available in 75 and 100 rounds versions.  Again, they are manufactured in both China, by Norinco, and in Europe.  These operate on a twisting spring mechanism, and must be wound to operate.  However, a fully loaded 100 round drum would give you a total 6 seconds of full auto fire.

Full Action Bipod (various)

Stock Set (various)

Sling (various)