Is there something like Gatherer but with inventory management?
To manage my collection, I have used deckbox.org. Since it uses gatherer as a "database" to know what all the cards are, it stays up to date with the current set. You will have to tell it what cards you do own however, so there is some up front work to tell the collection manager what you have.

It also uses magic.tcgplayer.com to let you know both the individual pricing of your cards and the contents of your entire collection.