What are the Features of a Good Birdhouse?
The Birdhouse Network of the Cornell Lab of Ornithology has a great list of the Features of a Good Birdhouse. Their good birdhouse features include: use a predator guard, put in drainage holes, no perches that can aid predators, the bird house should be made of untreated wood and many more good birdhouse tips. Another great resource is the Duncraft's Guide to Bird Houses. Duncraft's bird house features include: don't have a perch, check the entrance hole size for your preferred bird, you should be able to open the box so you can clean it out, and they have 7 other good tips on a good birdhouse.
More on Duncraft's Guide to Bird Houses.
What Birds use Bird Houses?
Duncraft has a good List of Birds that Live in Bird Houses. Not all birds are cavity nesters. So some birds do not use bird houses. The list above from Duncraft can give you a feel for cavity nesting birds to target with your birdhouse. Cornell Lab of Ornithology's website Nestwatch.org gives a good list of Cavity Nesting Birds Habitat Requirements. The list is near the middle of the page under the heading Requirements for Your Favorite Cavity-Nesting Birds. This lists the bird's name, the habitat requirement, bird house height and the entrance hole size. For example a House Wren needs a 1 1/4" round entrance hole. The Wren House should be 5 - 10 feet above the ground and House Wrens use a variety of habitats including farmland, open forest, forest edges, shrub land, suburban gardens, and backyards near trees or tall shrubs.
What are the Bird Specific Specs for Building Birdhouses?
About.com has a good list of Specs for Building Birdhouses that list 16 bird species and what size bird houses they prefer. Another good birdhouse spec list comes from Duncraft's Specs for Bird House Nesting Boxes. Duncraft lists 28 bird species and their preferred bird house size. These specs are important for deciding what bird house to buy or build for the needs of specific bird species you are trying to attract.