All front-line forces are armed with M16A2s and M4s and are issued M855 as standard-issue ammo. A few remaining Reserve and National Guard units, as well as some Air Force units, still carry M16A1s (you've probably seen them in the airports lately) and are issued M193 Ball (if they are issued any ammo at all) because of the difference in twist of the barrel.
Some special forces units, particularly in Afghanistan and Iraq, are using Mk262 and Mk262 Mod1 ammo. These are rounds loaded with heavy (up to 77 grain) JHP match bullets, in response to some issues with M855 terminal performance. This continues a recent trend towards heavier rounds (69 grains and over) for improved terminal ballistic performance.