Inventory website/app/software? I have several Lego sets, and I wanted to see if there was any app/software/website that I could enter the set number, and it would automatically inventory the bricks that I have. It would also be great if it had a catalog of builds that it could cross reference across my inventory, and show me things I could build now/could build if I purchased a few more bricks. 

Does anything like this currently exist?
Rebrickeable does what you are looking for. You can create an account and upload all of your sets by entering the set numbers. The website will automatically inventory all of your sets, and check what other sets and even custom models created by the community you could build with the parts you have. Here is the link to the website: https://rebrickable.com/

If you don't want to upload your entire collection but just want to quickly check what else you can build from parts of a few sets, you can check the combination of parts from up to three sets and see what you else you can build with it. This feature is under the BUILD tab: https://rebrickable.com/build/