Tuner Classic Movies (or TCM as it shows up in your channel guide) is pretty much the greatest channel that ever existed. It is without a doubt the best resource for people who enjoy classic films. I discovered TCM when I imposed on myself the task of watching every movie on these three best movie lists:
2)Time Magazine's 100 Best Movies -a great mix of current & old / foreign & American
3)Slant Magazine Essential Films -sort of an alternative to the conventional best movie lists. It includes lesser known films of beloved directors, a lot of silent film classics & indie movies.
All three lists contained a lot of films from 1920s-1950s that I had not seeked out before, and it turned out TCM played all of these movies on a regular basis. Soon I was swimming in great classics, discovering westerns & and falling in love with actors like Robert Mitchum and Montgomery Clift. I learned the history and evolution of movies in a way I never understood before. I soon began a ritual of downloading TCM's monthly calendar, and scheduling my life around movies I wanted to catch. (I don't do that any more, since I have TiVo)
My favorite experience with TCM happened on the night I decided to quit my wretched job in Baltimore. I stayed up til 4am and caught Marnie on TCM- a later work of Hitchcock that was dismissed as silly and insubstantial when it came out. I was introduced to a long list of Hitchcock films in college, but I had never heard of Marnie before. There were scenes in this movie that were so provocative and effective that it literally made my jaws drop. It's a movie that helped me appreciate Hitchcock on my own, as opposed to being told in classrooms why he was so great. To this day, I feel that it is an underappreciated classic, and I recommend it to every movie fan I meet.
I think most of the movies on TCM can easily be found on Netflix or your local video store. But it's so much more fun to stumble on to a movie you've never heard of before as I did with Marnie. I also totally L-U-V the short host segments before and after the movies. Robert Osborne and Ben Mankiewicz (yes, of that Mankiewicz family) are both fantastic hosts. They always highlight things I wouldn't have noticed, and are full of cool trivias and anecdotes. They also have these great mini segments between movies, where a current actor will talk about an actor from the old days, and why 
they love them. I especially love the one where Laura Dern talks about Shelley Winters, and also the one where Tony Curtis talks about being of fan of Cary Grant and then getting to finally work with him in Operation Petticoat. All the movies are shown uncut, uninterrupted, and in their original format. Yeah suck it AMC!