Some resources I've found recently...

Immigration Explorer-  The New York times provides an interactive map that shows census data and where specific immigrant groups were concentrated during that census.  The country can be looked at as a whole, or zoomed in and by states/regions.

Mapping History-  this site seems to be more geared for Middle School kids and would make a great resource for a web quest.  The site uses text and interactive maps to walk students through historical events.

Votetocracy-  This site posts what Congress is currently voting on and allows users to vote on the issue as well and see the trend of voting. The biggest lesson here would be the lack of drama on most things Congress votes on.  The site also provides data to users about trends in voting and breaks laws down by general topic. The site does require a username and password. 

Also found some of the best videos for use in world history that will totally click with students.  There's a post on my site with the link.... http://www.spotlight21.com