Howitzer

The 75mm howitzer is a pre-War weapons system that was used by the United States Armed Forces. A large number of these weapons was stored in various arms depots across the country, useless to anyone without proper training.

Post-War use
Algernon in New Reno has been provided with a howitzer by Eldridge for repairs (although what use could he have from such ordnance in his basement is unknown).

The Boomers acquired several of these guns from Area 2 to protect Nellis Air Force Base from enemies.

Caesar's Legion has acquired a howitzer for the Second Battle of Hoover Dam, but without a Howitzer Firing Mechanism, they've been unable to coax it to working order.

Appearances
A working howitzer and a slew of unusable ones appear in Fallout 2. Working howitzers appear in Fallout: New Vegas.

Location

 * Many howitzers can be found at Caeser's Camp, but they all seem to be broken when you try to access it.
 * There is one howitzer at Caeser's Camp that you can repair (Through a quest which is given to the Courier by Lucius inside Caeser's throne room) although you cannot use this, or any, of the howitzers, even though they are functional. They are only seen in use for the final battle at Hoover Dam.