Here are some links that are useful when discussing different kinds of media:
IMDB- The internet movie database. Has cast lists, quotes, and trailers for just about every movie (and TV show) ever made. The search function is also quite compresensive. Great for searching by movie or actor(actress).
Amazon.com- Well it's Amazon. It's got books and music, and some of the books are searchable inside the book. Another good place to look for links.
Wikipedia- A freely edited encyclopedia containing entries about everything.
Yahoo Movies- Another good cast listing site. Also has a pretty good upcoming movie section.
Apple Movie Trailers- Quicktime trailers of movies both current and upcoming.
Youtube- Handy for videos, clips, trailers, etc.
RottenTomatoes- I find its fresh/rotten meter is usually pretty representative of how enjoyable (at least I find) a movie is. Also, it's fun to tell someone his or her favorite movie got a dismal 5% fresh rating.
Allmusic- A music discussion resource.
Anything else anyone can think of?