If you don't have an exact LEGO model in mind and just want to discover something to build, use the Build Search page.
This page will search through every LEGO Set/MOC in the database, calculate your Build % for each, and show you the top results. Needless to say, this can be a very slow search! You can speed it up by changing the build options and filtering the search criteria to only the type of Sets/MOCs you are interested in.
You will typically only get 100% matches like above for MOCs that are Alternate Builds of a specific LEGO Set that you own, or if you have a ridiculous amount of LEGO! Clicking the Build button on a result will take you to the Build Results page where you can see the parts you are missing.
If there is a result you are not interested in seeing again, you can click the Ignore button to remove it from further searches.
In addition to the usual Build Settings, you can also modify the source parts used in the calculations. Use the two check boxes to include either or both of your My Sets and My Parts parts. In addition, you can add 1-3 extra sets just to see their affect on your build results. You can also use this to test building with only a single set as the source and uncheck your My Sets/My Parts collection.