Flags: This formula displays a green, yellow, or red flag image to indicate case priority.

IMAGE( 
CASE( Priority, 
"Low", "/img/samples/flag_green.gif",
"Medium", "/img/samples/flag_yellow.gif",
"High", "/img/samples/flag_red.gif", 
"/s.gif"), 
"Priority Flag")

Colors: This formula displays a 10 x 10 pixel image of a red, yellow, or green, depending on the value of a Case Age custom number field.

IF( Case_Age__c > 20, 
IMAGE("/img/samples/color_red.gif", "red", 10, 10),
IF( Case_Age__c > 10,
IMAGE("/img/samples/color_yellow.gif", "yellow", 10, 10),
IMAGE("/img/samples/color_green.gif", "green", 10, 10)
))

Lights: This formula displays a green, yellow, or red traffic light images to indicate status, using a custom picklist field called Status.

IMAGE( 
CASE(Status__c, 
"Green", "/img/samples/light_green.gif",
"Yellow", "/img/samples/light_yellow.gif",
"Red", "/img/samples/light_red.gif", 
"/s.gif"), 
"status color")

Stars: This formula displays a set of one to five stars to indicate a rating or score.

IMAGE( 
CASE(Rating__c, 
"1", "/img/samples/stars_100.gif",
"2", "/img/samples/stars_200.gif",
"3", "/img/samples/stars_300.gif", 
"4", "/img/samples/stars_400.gif", 
"5", "/img/samples/stars_500.gif", 
"/img/samples/stars_000.gif"), 
"rating")

Circles: This formula displays a colored circle to indicate a rating on a scale of one to five, where solid red is one, half red is two, black outline is three, half black is four, and solid black is five.

IMAGE( 
CASE(Rating__c, 
"1", "/img/samples/rating1.gif",
"2", "/img/samples/rating2.gif",
"3", "/img/samples/rating3.gif", 
"4", "/img/samples/rating4.gif", 
"5", "/img/samples/rating5.gif", 
"/s.gif"), 
"rating")

Priority: This formula displays a red colored priority sign to indicate a priority picklist value.

IMAGE( 
CASE(Priority__c, 
"Very Low", "/img/samples/rating1.gif",
"Low", "/img/samples/rating2.gif",
"Medium", "/img/samples/rating3.gif", 
"High", "/img/samples/rating4.gif", 
"Very High", "/img/samples/rating5.gif", 
"/s.gif"), 
"rating")