I can't believe no one's mentioned "Sullivan's Travels" yet. For shame!

Hell, just going through the Criterion Collection alone will keep you busy (and poor) for a good long while.

I just picked up "The Pocket Guide to Collecting Movies on DVD : Building an Essential Movie Collection-With Information on the Best DVD Extras, Supplements and Special Features-and the Best DVDs for Kids" by Steven H. Scheuer and Alida Brill-Scheuer, for $7.99 (I paid less at Borders with a coupon). This book reviews 134 major movies released on DVD, although not all fit your idea of classic. Each review discusses both the movie and any special features on the DVD. I think it's a really good book for getting ideas about great DVDs you might have missed. My only objection to the book is that they don't identify the DVDs they review by publisher catalog number or UPC, so you have to read the text very closely to figure out which version to buy if there's been more than one version released.

The Haunting
House of Wax

I can't believe that no one has yet recommended the AFI 100 lists.
If you want a good start for what the American Film Institute members consider the best of the best download any of their many 100 Best lists.
100 Years, 100 Hundred Films from 1998 spans all genres.

http://www.afi.com/tv/lists.asp

The AFI list is great if you're looking for strictly American films. I'd recommend also checking out the BFI "Sound and Sight" annual poll of directors and film critcs for a more diverse list.

Critics' Top Ten Poll

Directors' Top Ten Poll

I'd especially recommend checking out the list of all films receiving votes:

All films receiving votes