My first cat, Jasper, had an upside down heart on his chest. He was mainly white with black patches so the heart was black. I don't have any pics scanned of him (long time ago) but it was so cute!
I saw a cute red tabby and white cat on Catster that has 3 heart shaped marks on him. one on his side and 2 on his foreleg.
I used to have a cat ( blue lynx point and white domestic ) who had been very badly burned in a fire before we adopted him and when his fur grew back on his side, at the spot where his worst burn was, it came in with a mark of concentric circles like a bull's eye , with the circles in different shades of tan, and white in the center circle!
It did not stay like that forever though, it gradually changed and over the years ended up just a white spot .
Tan, with white chest and black stripey markings and green eyes Maine Coon-type cat (brother to cat above)
Solid black s/h, with a quarter-sized spot on his chest
S/h grey tabby, with black stipes and white chest and white mittens (sister to the Maine Coons)
2 torbies: grey, white, orange, black markings (one has one orange leg!)
S/h grey tabby, black stripes, white chest, with a rust-colored tiny elongated diamond on her forehead (loves having the others lick her there!). Sister to the torbies.
S/h grey with various shades of silver-grey "necklaces" on her chest (mom of the torbies and tabby, above)
S/h mostly white, with dark grey patches (MargeCat!!!!)