I just recently signed up for O'Reilly's "Safari" on-line bookshelf. I needed to learn more about Linux networking (more than I wanted to know, really, but que sera). I was pleasantly surprised to find that Safari now has Addison-Wesley books available as well as O'Reilly books (and I have no idea how long this has been the case). That really makes the service much more interesting.
O'Reilly publishes a lot of good books, but it has its niches, and they aren't always the niches I'm interested in. For example, they have books on Linux networking, but if you want something about EMF (the Eclipse Modelling Framework), you're out of luck. Addison-Wesley have one, though, and that book ("Eclipse Modelling Framework: A Developer's Guide) is available on Safari.
Two publishers are definitely better than one.