Eli (Denzel Washington) carries a weathered Heckler & Koch HK45 pistol (an LEM variant as noted by the bobbed hammer and lack of external safety) as his sidearm throughout the film. In true movie fashion, Eli fires about twice as many rounds as the pistol can hold before reloading.
Trivia: Even though the pistol had only been out a year when the film was made, the brand new HK45 was sanded and worn down to look very old (as this film takes place around 3 decades in the future, so it will look like an antique vintage firearm) for the film. The actual pistol is currently on display at the National Firearms Museum located at the NRA headquarters in Fairfax, VA.
Redridge (Ray Stevenson) is ordered at one point to shoot Eli with his two toned Beretta 92FS pistol.
Colt New Service
What appears to be a Colt New Service revolver is seen at the beginning of the film, lying on the ground, unloaded, in the middle of the woods. It is never held or fired, however.
Shotguns
Remington 870 "Witness Protection"
Eli (Denzel Washington) carries a short barreled Remington 870 "Witness Protection" shotgun, with various weathering and a heavily taped handle. In the end of the film, Solara (Mila Kunis) can be seen with the shotgun, as she "inherits" it from Eli.
Remington 870
Hoyt (Joe Pingue), one of Carnegie's men uses another Remington 870 with magazine tube extension and saddle shell holder. He is seen most notably at the end loading his shell holder as anarchy breaks out at Carnegie's bar near the end of the film. Other men are seen with different Remington 870's.
Mossberg 500
Another one of Carnegie henchmen uses a Mossberg 500 with wooden furniture.
The Engineer (Tom Waits) holds a Sawed-Off 12-Gauge Double Barreled Shotgun when Eli comes into the store. Eli quickly disarms him of it, but returns it as a sign of trust. One of Carnegie's men also uses one after the fight at Carnegie's bar.
Rifles / Carbines
AK-47
Some of Carnegie's thugs carry AK-47 variants, most likely Norinco Type 56 rifles.
FN FAL
Redridge (Ray Stevenson) carries an FN FAL paratrooper as his main firearm in the film. Also one of Carnegie's other thugs uses one during the shootout at George's house.
Colt M4A1
George uses an M4A1 Carbine fitted with a vertical foregrip and Aimpoint sight at one point during the assault on his house. The guards at Alcatraz also use M4A1s fitted with red dot sights.
M16A2
Some of Carnegie's men wield M16A2 rifles several times in the film.
M1903 (Mannlicher Schönauer Carbine stock)
When Eli is walking through the town the camera pans on a man on a roof with a bolt-action rifle. This rifle is a sporterized M1903 Springfield with a Lyman peep sight and fitted into a Mannlicher Schönauer Carbine (Stutzen) stock. The military stock had been replaced, as well as the sights, and likely the parkerized finish was replaced with a blued finish. This was common practice at a time when surplus military rifles were cheaper than sporting rifles. It is the same prop featured in the film A Boy and His Dog, noting the heavy stylistic and atmospheric homage already paid, as well as featuring a poster of the movie.
M1 Carbine
One of Carnegie's men holds an M1 Carbine during the standoff outside the bar and the shootout at George and Martha's house.
Blaser R93 Sporter
What appears to be a Blaser R93 in sporting configuration is held by one of Carnegie's men.