A sea urchin hitch-hiked into my tank with some live rock in 2006. At first it was so tiny that it was a couple of weeks before I noticed it (it was about the size, if not smaller, than a plain m&m candy).
It would hide in tiny crevices with only its spines sticking out:
When it got a little larger it began to roam:
Gradually it put on some size:
A little over a year later in 2007 (and golf ball-sized) I had to transfer it in my Fish Only system since it started toppling over corals in my tank (including one of my trumpet corals which is nearly the size of a grapefruit, I didn't think anything could move it). Here it is on the glass in search of coraline algae to eat:
One of the interesting things about it is it typically hides in one particular spot during the day, roams around the entire tank at night, and somehow returns to its spot again before the lights come on.