Nice excerpt from American Lung Association page

At 20 minutes after quitting:

blood pressure decreases
pulse rate drops
body temperature of hands and feet increases

At 8 hours:

carbon monoxide level in blood drops to normal
oxygen level in blood increases to normal

At 24 hours:


chance of a heart attack decreases

At 48 hours:


nerve endings start regrowing
ability to smell and taste is enhanced

At 2 weeks to 3 months:

circulation improves
walking becomes easier
lung function increases

1 to 9 months:

coughing, sinus congestion, fatigue, shortness of breath decreases
1 year:

excess risk of coronary heart disease is decreased to half that of a smoker
At 5 years:

from 5 to 15 years after quitting, stroke risk is reduced to that of people who have never smoked.

At 10 years:

risk of lung cancer drops to as little as one-half that of continuing smokers
risk of cancer of the mouth, throat, esophagus, bladder, kidney, and pancreas decreases
risk of ulcer decreases

At 15 years:

risk of coronary heart disease is now similar to that of people who have never smoked
risk of death returns to nearly the level of people who have never smoked