Most of us know that you can't find everything on the Internet. We also know that WorldCat contains records for over 1 billion books, videos, scores, etc. from libraries all over the world. What I often forget is that not everything is in WorldCat either.
Now we have the Not in WorldCat blog which focuses on items that, for whatever reason, haven't been cataloged in WorldCat but are available for sale online. I like being reminded that there are things out there just beyond my reach.