I just moved out into the country and near a lake and we have lots of spiders with white sacs that come out only at night and hide under a web blanket during the day. What kind of spider is this?

If they have white sacs, then you are going to have many,many baby spiders soon.

Impossible to know what type of spider they are with so little information. You might try finding your spider on this web site:

What's That Bug? (http://www.whatsthatbug.com/)