Many schools have extensive demonstration website were many useful ideas can be found. A few useful ones are listed below. Each of them has links to many other great sites.
North Carolina State University
Universoty of Southern California
Michigan Technological University
A great search engine can be found at http://physicslearning.colorado.edu:9999/vestris/QuerySp.html that indexes 39 different demonstration sites.
The Physics Instructional Resource Association (PIRA) has compiled a list of many standard physics demonstrations and an indexing system (DCS) to give a standard name to the demonstrations. Members of PIRA also maintain a webring of websites that support physics teaching. PIRA members also maintain a listserve (TAP-L) that support discussion of physics teaching.
Virginia Tech Physics Lecture demo list