In Ultima I: The First Age of Darkness, if one created a thief character and through racial and point allocations increased your Dexterity to a high level, one could obtain the two most powerful pieces of equipment in the game, the Reflect Suit and the Phazor, by attempting to steal from the armor and weapon shops of any store (Even if they weren't purchasable at the time), thus obtaining a sizable amount of money (By selling their own goods back to them) and end up with the most powerful gear before you even fight your first creature or enter your first dungeon.
In Ultima V: Warriors of Destiny, if the player knows where to get the flying carpet, they can get 5 skull keys, that can be used open any magically locked door in the game, for 100 gold from a NPC. Normally this NPC only has the 5 skull keys to sell, but by exploiting a bug in the game, the player can keep on buying the skull keys indefinitely (assuming they have the money). The keys can be used to break into the storeroom in the basement of Castle British and stealing all kinds of good equipment. Since the chests "respawn" every time the Castle is reloaded, the player can somewhat easily get very good equipment and tons of money. This does play havoc on the party's Karma, though. But this is easily fixable by donating money to the shrines.
In Ultima VII: The Black Gate, if you start the game from the command line with "ultima ABCD[Alt]255" (hold down [Alt] and press 255) and you will get a cheat menu.
There is a house southwest of Trinsic, the starting town, you can line up crates to form a stair to the rooftop, if you go behind the chimney and them south, you will end up on The Cheat Room. It has 8 suits of Magic Armor, 2 Death Scythes, a Juggernaut Hammer, a Bird (it's a weapon), 2 Magic Axes, lots of Magic Rings, a Spell Book with all the Spells, 1000 Gold, 100 Lockpicks, 100 of each Reagent, a Purple Wand, three Prisms and a Hoe of Destruction. There are also teleporters to most all of the major locations in the game plot as well as nearly every item critical to finishing the game.
If you you know the way, you can get the Magic Carpet, which is not only free, but also allows you to travel almost anywhere in right after getting in Britain, the third city in the game.
You can also get the Hoe of Destruction, the second best weapon in the game, right after getting in Britain.
The expansion, Ultima VII: Forge of Virtue, containins a dungeon whose payoff is the Black Sword, a weapon far more powerful than any in the original game, as well as boosting the main character's stats to the maximum (twice the maximum in the case of the Strength stat). Although the main character doesn't discover how to find the new area until about halfway through the main game, if the player knows where it is they can access it almost immediately.
In the Test of Love, flip the switch next to the Stone of Castambre. Then go back into the caves to the north. A secret passage will have opened that leads to a room with a Death Scythe, 9 Glass Swords and a Firedoom Staff.