I grew up calling these hens & chicks, but in older texts it is referred to as houseleek. Their use as a living roof material is still practiced in Slavic nations, and dates back to at least early medieval times. Historically it was believed to protect a dwelling from lightning and fire. This succulent is originally indigenous to the Alps and is considered edible in small quantities.

Photo taken June 9, 2004.

Same plant as above, but with many babies from the season.
Photo taken November 6, 2004.

Same plant as above. Photo taken June 14, 2006. 

Go figure, the darn things actually bloom! Photo taken August 23, 2006.

Close up of the rather insignificant looking bloom. Photo taken August 23, 2006.