A few months ago we pointed out the great house styles guide featured on About.com, but we wanted to highlight a few other informative resources for determining house styles.
Modern House Styles [Oldhouses.com]
Historic Style Guide [Antique Homes Magazine]
Browse by Style [Houseplans.com]
>Historic House Architecture [Ragtime]
Abrams Guide to American House Styles
American House Styles
Field Guide to American Houses