BAT CONSERVATION INTERNATIONAL
A well-known organization devoted to the conservation of bats,

BAT WORLD SANCTUARY AND EDUCATIONAL CENTER
An organization dedicated to saving bats and their habitats.

THE BUZBEE BAT HOUSE
Great site, featuring details about the author's experiences with bat houses as well as an enormous compilation of links to all sorts of other bat pages and sites.

SPECIFIC PRACTICAL ADVICE:

How to get bats out of your house [without harming them]:
http://www.batcon.org/discover/unguest.html

Information on rescuing bats and what to do if you've found an injured bat can be found at the Bat World site.