Wolf spider, Pardosa sp. carrying her egg sac (31/05/2011), attached to her abdomen.
Nursery web spider carrying its large egg sac (19/06/2011).
In the family Gnaphosidae, the female not only guards the eggs, but she makes her egg sac inside a folded up leaf, locking herself inside with the eggs (19/07/2010):
But egg sacs are often found alone. Some of them have a stalk, like those of the genus Ero, also distinctive by their covering of coarse red silk (7/04/2013):
These are just a few examples, from common spiders you can find in your own garden, which illustrate some of the diversity of spiders.