Old Faithful Geyser may be the best known geyser in the US.  Old Faithful is not the most regular nor the tallest geyser in Yellowstone.  But its combined size and regularity make it the most popular.  Its eruptions occur on average every 92 minutes, though intervals can vary from 30 to 123 minutes.  Its height can exceed 180 feet.  Given its size and regularity, Old Faithful is a great photo subject.  Exploring eruptions under varying lighting can be a lot of fun.  This page contains a collection of images of Old Faithful at various times of day (and night), both in eruption and not.