For a lovely bonus in the single-player game, toss some coins in the bard's tray (in the center of town).
When playing as the thief, you can steal from the chickens in town. Your reward is entirely appropriate. :-)
The fastest road to easy money is by starting a new game with two monks. Transfer both the money and book of resurrection to one monk then quit the game. Start a new game with that same monk and another new monk, repeating the process until inventory is full of books and money. Ideally transfer books to a fighter with a good trade skill and sell all the books bringing in a total of about 180,000gp. Deposit gold and repeat. Mmmm.
Easy $1 Million Gold
Take 20,000 gp and throw 10,000 on the ground in front of you. Now fill your inventory up with all kinds of stuff so it is comletly full. Right click on the 10,000 you have in your inventory and remove 9999, put them on the ground, then close the inventory screen and try to pick up the 10,000 you put down earlier. He drops the money on the ground again but if you look in the inventory the gold has counted from 1 gp to 10,000 gp then you just remove 9999 gp from the inventory, close screen, pick up the 10,000 on the gound and then repeat this trick over and over. Soon you will have 1 million gp.