Pathfinder Online lets you type a callsign once and search many web sites for QSL information; if there's an online callbook for the callsign's country, it too can be searched with a single mouse click.
I recently sprinked some Web 2.0 dust on Pathfinder Online: a user-driven rating system for QSL info web sites, and a sidebar with one-click access to useful DXing resources. Suggestions for additional DXing resources would be appreciated.
Pathfinder Online is available via http://www.dxlabsuite.com/pathfinder/WebClient/
The boring old look is still available via http://www.dxlabsuite.com/pathfinder/WebClient/DustFree.htm
And because two choices are never enough, there's http://www.dxlabsuite.com/pathfinder/WebClient/NoRatings.htm