http://www.antarctic.com.au/enyclopaedia/EA.html - Slight Australian bias, excellent graphics and exploration section.
http://www.walcot.demon.co.uk - A little gem of a site.
http://www.spri.cam.ac.uk - The Scott Polar Research Institute, Cambridge University. Friendly and unassuming, excellent bibliography section. Other sites have better educational sections.
http://www.south-pole.com/homepage.html - Full of historical information.
http://www.heritage-antarctica.org - Small but interesting, but when does pollution become a museum piece?
http://www.mnc.net/norway/Amundsen.htm - Amundsen information.
http://www.pbs.org/wgbh/nova/shackleton/surviving/quest.html - In depth (for a web page!) report of Shackleton.