To make the Cheat Gnome appear in front of your house, while at home hold B and press Up, Left, Down, Right. Select the Gnome and you will be given 9,999 Simeoleons. This can be repeated up to 999,999 Simoleons.

To move time six hours forward, unlock the Cheat Gnome and then press Right(5), Left.

To unlock an option to change skills, unlock the Cheat Gnome and then press Down(3), Up(3).

Activate the following cheats by entering the corresponding codes at the "Game Options" menu.

Collars
Black Dot Cats - ZZZ11ZZZ9Q11Q91ZB1A
Black Dot Dogs - ZZZ11ZZQHZ11Q91ZB1A
Black Smiley Cats - ZZZ11ZZQ9Z1AQ91ZB1A
Black Smiley Dogs - ZZZ11ZZZ9QAAQ91ZB1A
Blue Bones Cats    - ZZZ11ZZQHZAAQ91ZB1A
Blue Bones Dogs    - ZZZ11ZZZHZ11Z91ZB1A
Blue Camouflage Cats - ZZZ11ZZZHQ11Q91ZB1A
Blue Camouflage Dogs - ZZZ11ZZZHZ1AQ91ZB1A
Blue Star Cats - ZZZ11ZZQ9Z11Z91ZB1A
Blue Star Dogs - ZZZ11ZZQHQ11Q91ZB1A
Green Flower Cats - ZZZ11ZZZ9ZA1Q91ZB1A
Green Flower Dogs - ZZZ11ZZQHZ1AQ91ZB1A
Navy Hearts Cats    - ZZZ11ZZZ9Z11Q91ZB1A
Navy Hearts Dogs    - ZZZ11ZZQ9Z11Q91ZB1A
Orange Diagonal Cats - ZZZ11ZZQ9ZA1Q91ZB1A
Orange Diagonal Dogs - ZZZ11ZZZ9Z11Z91ZB1A
Pink Vertical Strip Cats - ZZZ11ZZQ9QAAQ91ZB1A
Pink Vertical Strip Dogs - ZZZ11ZZZHZA1Q91ZB1A
White Paws Cats    - ZZZ11ZZQ9Q1AQ91ZB1A
White Paws Dogs    - ZZZ11ZZZ9Q11Z91ZB1A
White Zebra Stripe Cats - ZZZ11ZZZHQ11Z91ZB1A
White Zebra Stripe Dogs - ZZZ11ZZZHZ11Q91ZB1A

Fur colors
Blue Cats    - ZZZ11ZZQHZA1Q91ZB1A
Blue Dogs    - ZZZ11ZZQ9Q11Z91ZB1A
Deep Red Cats - ZZZ11ZZQ9QA1Q91ZB1A
Deep Red Dogs - ZZZ11ZZQHQ1AQ91ZB1A
Goofy Cats    - ZZZ11ZZQHQ11Z91ZB1A
Goofy Dogs    - ZZZ11ZZZHZAAQ91ZB1A
Green Cats    - ZZZ11ZZZ9QA1Z91ZB1A
Green Dogs    - ZZZ11ZZQHQA1Q91ZB1A
Light Green Cats    - ZZZ11ZZZHQ1AQ91ZB1A
Light Green Dogs    - ZZZ11ZZZ9Q1AQ91ZB1A
Neon Green Cats - ZZZ11ZZZHQA1Q91ZB1A
Neon Green Dogs - ZZZ11ZZZ9QA1Q91ZB1A
Neon Yellow Cats - ZZZ11ZZZHQAAQ91ZB1A
Neon Yellow Dogs - ZZZ11ZZQ9QA1Z91ZB1A
Pink Cats - ZZZ11ZZQHZ11Z91ZB1A
Pink Dogs - ZZZ11ZZZ9Z1AQ91ZB1A
Purple Cats - ZZZ11ZZQ9ZAAQ91ZB1A
Purple Dogs - ZZZ11ZZQ9ZA1Z91ZB1A

Fur patterns
Bandit Mask Cats     - ZZZ11ZZQHZA1Z91ZB1A
Bandit Mask Dogs     - ZZZ11ZZQHQAAQ91ZB1A
Panda Cats    - ZZZ11ZZQHQA1Z91ZB1A
Star Cats - ZZZ11ZZZ9ZAAQ91ZB1A
Star Dogs - ZZZ11ZZZ9ZA1Z91ZB1A
Zebra Stripes Dogs - ZZZ11ZZZHQA1Z91ZB1A