The Facebook Graph API is great and in one year has thoroughly upgraded Facebook's approach to its platform. The Graph API is much simpler than previous iterations of Facebook's APIs and Connection methods. Of course, with the simplicity of the Graph API, it's still easy to get overwhelmed by just how much information you can easily (with explicit and secure user permission, mind you) get access to and use in your apps. Facebook employee Simon Cross put together a handy Facebook Graph API Explorer to help you navigate the Graph API response content.
Similar to Gowalla's API Explorer, you're able to specify an API endpoint, hit a button and see the full JSON response that your app would get when making calls. Even handier, you can enter or login to get your access token and see even more specific data. You can even do POST requests and add parameters to experiment with making posts to the Graph API.
It doesn't stop at just the Graph API, either. You can also practice or debug FQL (Facebooks YQL-like query language) queries. The Explorer even lets you see some example queries, if you happen to be new to FQL.
The Facebook Graph API explorer is definitely worth keeping in the toolbox/bookmark folder if you work with the Facebook API. Hopefully, Facebook will integrate this more officially in to their own Developer tools and continue to expand on it. And luckily, they already employ the guy who built it.