I spend a fair bit of time digging around digital archives. Even compared to a just a couple of years ago the amount of stuff, especially on-line, is mind-boggling.
Something cool announced this week: on November 1st, 2009 National Geographic is releasing a 6 DVD set of it's ENTIRE archive going back to October 1888 !! It's technically a re-release as NG did something similar in 1997 but backed down to enter into a legal battle over the rights of a publisher to publish free-lance work.
On a related note, her are some digital archives that I sometimes dig into. The full list can be found on the sidebar of my history blog.
Virtual Heritage Winnipeg
Winnipeg Tribune Archives
Alberta Heritage Digitization Project
Peel's Prairie Provinces
Library and Archives Canada
Smithsonian Online Collections
Library of Congress Image Collection
UNESCO World Digital Library