I am building an inventory of all my Lego pieces and I am looking for a website that can give the pieces in a set with only the set number. It would be easier if there was information for many Lego sets with the full inventory. I also will need to have a file of all Lego parts.
Here are the options that I'm aware of along with some basic pros and cons:
This site is great for old sets, and has some nice functionality including the ability to compile a list of all the parts in all of your sets for you have. This makes it easy to start your part inventory. All you have to do is import your sets and then export your parts list.
Unlike Peeron, Bricklink is getting updated constantly, so it shouldn't have issues with newer sets. These inventories are also very detailed and include extra parts, sticker sheets, etc. If you are a seller on Bricklink, you can also part out sets into your inventory which could be useful to you at some point.
Brickset / Lego.com
TLG provides nearly complete inventories for sets online via the replacement parts page. You can use this to get part information from your sets, but the interface is lacking because this isn't really what the page is meant for.
Brickset provides this same part data in a format which is much better for viewing. Simply search for a set and use the "Parts" tab to view contents if they are available.
These resources have a couple of downsides:
Rebrickable also provides set inventories. Once you search for a set, you can scroll down to find a list of parts. This source includes sets from all time, and is still being updated as new sets come out. Rebrickable also provides set inventories for offline use via its download page.
Try the bricklink catolog. They also have inventories like Peeron, but a little bit more detailed (like part variations) and also include stuff like extra parts and Ninjago cards.