A large number of online databases are hidden behind the web. Users to these systems can form queries through web forms to retrieve a small sample of the database. Sampling such hidden databases is widely desired for understanding the nature and quality of data stored in them. We have developed HDSampler, which to the best of our knowledge is the first practical system for sampling structured hidden web databases. It enables efficient sampling of the databases and accurate answering of aggregate queries, to provide analysts with valuable information for data analytics, as well as help power a multitude of third-party applications such as web-mashups and meta-search engines. For the purpose of this demo, we present an instance of HDSampler on Google Base a content-rich hidden web database maintained by Google. By using HDSampler, the demo reveals a snapshot of the marginal distribution of various attributes of Google Base in a matter of minutes.