Beta Weapons are weapons that were not included in the game. It is also includes some weapons that didn't make it to the final game and weapons have their models changed. Many made well during development and it is unknown why they were removed. However, very most of them can be obtained through cheat devices and trainers.
Grand Theft Auto 2
Weapons called electro-baton, instant vehicle bomb and microwave were cut. The first two have their HUD icons inside game files.
GTA Vice City
AK-47 was cut before the release of the game. Landmines were cut.
Grenade Launcher was also cut. It is said that the
Tazer was also included in the game, but was also cut. The
Nailgun was believed to be included in the game. Deep inside the PS2 version there are some remaining files (model, texture) of Steyr Aug.
PSG-1 never had a silencer. The
MP5 looked shorter. The
M4 in the retail version was originally an M16A2. Tear Gas was removed from Xbox, PC and later PS2 versions of the game.
GTA San Andreas
Skateboard was cut from the PC version. It was a melee weapon. Available as a mod for the PC version but is already usable on the PS2 version. The
M47 Dragon was cut. The
M82 was cut. The Grenade Launcher was cut again in the game.
M249 SAW and M60 were cut.
The Micro SMG known from GTA VC was also removed, but can be seen in several early screenshots.
GTA Liberty City Stories
There is existing tear gas, but it can be only acquired via trainers.
GTA Vice City Stories
tear gas and landmines were going to be included in the game. However, they can be obtained through trainers. The
phone was cut and replaced with a pager. In the PSP version it's possible to take the phone out with cheat devices, but it's useless. The
Scorpion was originally shot two-handed.  The Camera was originally available outside of missions and would've been found behind a building in Ocean Beach.  
The silenced 9mm pistol, revolver, MP5, flamethrower, nightstick, wooden bat, pool cue and minigun were to be included in the game.
The Ballad of Gay Tony
Error creating thumbnail: File missing The beta version of the Assault SMG. Notice the lack of a silencer.
Armando Torres would have be able to sell every weapon in the game from his gun van including the Desert Eagle and the Micro Uzi. The
Automatic Shotgun was fully automatic. The
Advanced Sniper's scope was not tinted green. The
Assault SMG did not have a silencer. The Parachute was all yellow in the beta, instead of yellow, white and blue.
Advanced Rifle originally looked a little different.  The
Combat MG originally looked very similar to TBoGT's Combat MG.  The
Assault SMG originally looked much more like a real FN P90.  The Heavy Rifle, a SCAR-like rifle, was cut.
 The Assault Sniper, an M110-like rifle, was cut.
 The Assault MG, based on the HK21, was cut.
 A smart gun called the Programmable A.R., based on the XM-25, was cut.
    The Wrench was going to be a usable melee weapon.
 The Fire Axe was going to be a usable melee weapon.
 The Shovel was going to be a usable melee weapon.
 A Loudhailer was cut, it was most likely a mission only weapon.
 The Lasso was cut.
 A Rubbergun was cut, which was very similar to a  Micro SMG and shot rubber bullets.  
The Laser weapon is unused.
 The Hunter weapon is unused.
 The Dune weapon is unused, it is a .50 cal machine gun.
 The Hind weapons were cut, both Gun and Rocket.
 The Vulkan weapons were cut, both Guns and Rocket.
 The Submersible weapons were cut.