Where can I find a list of films that are "public domain" (copyright expired) and can I show them in my films?

Wikipedia actually has a decent reference guide to post-1923 films:

http://en.wikipedia.org/wiki/Category:Public_domain_films

Also, try this website:

http://www.thoughtequity.com/video/home.do?gclid=CM692f6C7IwCFQ8_UAodDVmo1A