Snowball Earth
Good article on Snowball Earth/Earth's ice ages:
http://news.bbc.co.uk/2/hi/science/nature/2163646.stm Good links on the topic to the right of that page:
11 Dec 98 | Science/Nature
Meteorite is possible ice age culprit.
18 Oct 01 | Science/Nature
Antarctic cores reveal ice history.
06 Mar 02 | Scotland 'Snowball Earth' theory melted.
There was a similar discussion here at the HZ:
http://www.habitablezone.com/space/messages/22421.html