MOC Pages
MOC Pages is a great place to see what others are doing, post your work and learn...alot.
Brick Link
A great place to buy and sell Lego bricks or sets. Find everything you need to build your MOC or get lost pieces.
Flickr
Lots of great stuff here and people who like to share the photos that really show off all the details of their work.
Stud.io
Bricklink's Stud.io software directly connects and lets you know if the part actually exists. It will even tell you how much each part is currently going for and the estimated total for your creation.
Pintrest
Here is a link to my Lego Awesomeness board on Pintrest. Good stuff collected from around the web. Good to browse on mobile devices with the app.
LDD
Lego Digital Designer is a great place to start building your MOCs. I can stress enough how great this has been in helping me create new great builds.