There is a relatively new movement called NoSQL. It involves high performance databases for little to no cost. Social Networks have been using NoSQL. You might be able to use them too, depending on your specific database requirements.
The NoSQL movement has different products from different vendors. Amazon created SimpleDB. It is data storage for key value pairs. And Google created BigTable, open sourcing the data model for it.
Some NoSQL databases focus on storing JSON objects. Examples of these databases are CouchDB and MongoDB. CouchDB let's you use a REST API to access the JSON data.
These NoSQL databases often have limitations. Some impose a time limit for queries to complete. Others return only partial data sets. These databases can hold a lot of data. But they often are poor for complicated queries.
Hot Topic Spamming Failures - I follow a friend into a Hot Topic store in the mall. Saw some cool merchandise. Decided to buy something. The salesperson asked me a bunch of questions, ...