All the military personel seen throughout SG-1 are Air Force, unless otherwise stated. We know there have been a few Marine SG teams, most notably the SG-3 Marines.
Interestingly enough, although the Air Force runs the American Stargate program, the Russian one was run by Russia's Army, with an Army officer in command of the Russian starship Korolev.
In Atlantis all the military personel in the first season were Marines with the exception of Sheppard, who was Air Force. Colonel Sumner identified himself as a Marine, as did Colonel Everett in the finale. Likewise Lt. Ford and several of the support characters clearly had USMC on their uniforms. Strangely enough, Sgt. Bates had USMC on his uniform, but when they brought him back in the fourth season, he spoke of being discharged from the Air Force. In subsequent seasons more Air Force personel were brought in (Daedalus crew, Major Lorne). Also, the episode Runner established that all branches of the US military had a presence on Atlantis.
Most of the SGU characters are Air Force, though there is a Marine presence there as well. Sgt. Greer is confirmed as a Marine, and I think Sgt. Spencer was as well. Though Colonels Young and Telford, Lieutenants Scott, Johansen, and James, and Sgt. Riley are confirmed as Air Force.