Other
Washington University: Polyatomic Ions
This page from the Department of Chemistry at Washington University provides attractive Lewis dot diagrams and three-dimensional models for many of the common polyatomic ions. The images will make your visit worthwhile.
University of Massachusetts
University of Massachusetts: Rasmol & Chime
This site from the University of Massachusetts links to downloadable freeware (Rasmol and CHIME) which are required to view 3-D images common on organic chemistry tutorials.