It's not just cow skulls painted in gaudy southwestern colors anymore. Collections of skulls, long considered mainly the province of the scientist (mad or otherwise), are gaining popularity. This is due to many factors, including the trend towards Native and Latin American decorating styles, the resurgence of amateur nature appreciation and study, and the Gothic revival. Though there have been times and places where a collection of the skulls of one's ancestors or enemies was the ultimate decorating statement, the current legal climate is such that most collections are composed entirely of animal skulls.
But that's okay, because many animal skulls are wondrous things in their own right. Click here to see a random weird skull available from Skulls Unlimited International.
Those wishing to start their own skull collections should observe special cautions. Skulls found in nature can potentially carry disease and should be carefully cleaned. (Gerald Durrell's The Amateur Naturalist provides instructions for several methods for cleaning.) If you are collecting skulls from road or railroad kill, let caution temper your enthusiasm; I know of at least one person who became roadkill herself while engaged in roadkill examination. And whether you find or purchase your skulls, remember that the possession of parts of endangered species without a special permit is greatly frowned upon by the law of the land.
Resources for those with skulls on their minds: