JLPT Practice - Home
They allow you to take tests that they have made online (the system is really good! Just like the real thing!), and you can see the answers right away. They even have the listening sections of the test!
They have tests for JLPT level 2, 3 and 4.
They also have many links and resources (practice sheets, etc...) for taking the JLPT and a bit for Japanese in general.