Robert Neville (Charlton Heston) carries a Smith & Wesson M76 9mm Submachine gun throughout the film. The Weapon is also wielded by Lisa (Rosalind Cash).
MP40
Robert Neville has a German MP40 in his gun rack at his penthouse. It is amongst the guns smashed to the ground by Mathias' followers, but Neville manages to grab it and kill several mutants. A pivotal event in the film is when Neville has a chance to kill Mathias, but his gun jams (possibly a shell separation in the chamber or dud ammunition, since the open bolt mechanism works fine).
Thompson M1A1
There is one scene that an M1A1 Thompson Submachine Gun can be seen strapped to the back of Dutch's motorcycle seat. Also an M1A1 is seen on Neville's Gun rack in his apartment.
Colt 1903/1908 Hammerless
The first time that Neville meets Lisa (Rosalind Cash) she is pointing a Nickel plated Colt 1903/08 Hammerless with Pearl grips at him. Unknown if it is the 32 or 380 caliber model. Note: In the very first scenes, Lisa's pistol is a silver painted rubber gun, probably the stunt weapon she handled during the night filming and the subsequent motorcycle chase.
M1911A1
Robert Neville carries a Colt M1911A1 in a brown holster throughout the film. He never draws nor fires it, but several M1911A1s are visible on a gun rack in Neville's Apartment.
Star Model B
Zachary (Lincoln Kilpatrick) tries to use a Star Model B Pistol to assassinate Neville with bad results.
Single Action Army Revolver
Dutch (Paul Koslo) carries a nickel plated Single Action Army Revolver with ivory grips. This is most noticeable during the night time rescue of Neville at the Los Angeles Coliseum and later when Dutch rides up in his Motorcycle at the sanctuary.
Browning AN/M2 machine gun
When Neville arrives at the other survivors hilltop hideout he is covered by Tommy. Tommy is using a Browning AN/M2 Machine Gun with a Browning 1919A6 Buttstock on a tripod.
Browning M2 Aircraft Version
M3 Carbine
Apparently the gun that the IR scope was taken from, there is an M3 Carbine sans scope (with the fore grip and electrical wiring attached) sitting on one of Neville's chairs in the apartment near the window. The M3 Carbine is also later seen in the gun rack.
Ithaca Model 37 Shotgun
Completely incidental and briefly seen, a dead Los Angeles Police officer lies near his Ithaca Model 37 pump shotgun, a victim of the plague It is hard to see since the shot is a fast zoom to the dead man's face. Trivia: The actor playing the dead L.A. cop can be seen breathing when the camera lingers on him.
Colt Official Police
A Colt Official Police (.38 Special) is incidentally seen on the seat of an LAPD patrol car, the officers dead from the plague.
Robert Neville's Gun Racks
A variety of weapons that are not seen or utilized anywhere else in the film are seen here.
Several rifles which appear to be M1903 variants are on the rack but never fired or handled. One of them is briefly seen with a scope. Hard to see if it is an 03-A3 or an O3A4, but the scope is a commercial one.
Trivia Special
Early Flashlight mounts
As in other films before the advent of integral flashlight mounts for submachine guns and other firearms, shooters had to make do. The DIY crude metal clamps that Robert Neville uses to affix a large and bulky flashlight to his M76 Submachine gun was the precursor to the Surefire (and others) tactical lighting systems we enjoy today.