After spending a year in private beta, Amazon's SimpleDB has been pushed into what the company calls "unlimited public beta." Any developer or business can now sign up and start using the web service.

As a come-on, for the next six months or so Amazon will be offering a free monthly usage tier good for 25 machine hours, 1GB of data transfer in and out, and 1GB of storage.

Exceed those limits and additional machine hours cost 14 cents, along with 10 cents a gigabyte transferred in and 17 cents a gigabyte out up to 1TB.

Amazon's also cut the price of structured data storage by ~83% from $1.50 to 25 cents a gigabyte a month.

During the beta, users can have up to 100 domains and 10 gigs of data in each domain. That's 1TB of data in all.

SimpleDB, which as the name suggests is simpler than a traditional relational database, provides the core database functions of indexing and querying. It requires no schema, automatically indexes data, and provides simple APIs for storage and access.

Amazon compares a SimpleDB domain to a spreadsheet, with items like rows of data, attributes like column headers, and values the data entered into each of the cells. Unlike a spreadsheet, however, SimpleDB allows multiple values to be associated with each "cell" and each item can have its own set of attributes.

Naturally SimpleDB integrates seamlessly with Amazon's other cloud services, Simple Storage Service (S3) and Elastic Compute Cloud (EC2), to provide an infrastructure for web-scale applications.

SimpleDB is supposed to let businesses and developers easily scale database processing and free developers from the complexities of capacity planning while cutting large capital expenditures into smaller operating costs.

Amazon says it would take a clustered RDBMS to do the job of SimpleDB although SimplexDB, which uses less dense drives than, say, S3, is better with smaller data elements and is not for storing raw data.

Amazon is particularly proud of SimpleDB's concurrent access. It says it "really shines" and hit 5,600 requests a second during the private beta.

Customers can now get Premium Support for SimpleDB, which means technical and operational assistance from the Amazon Web Services team whenever and as frequently as they need it 24x7. Amazon says they can count on predictable response time and personalized support.

Reportedly in response to customer feedback, Amazon Web Services is planning to give SimpleDB a couple of new features in the next few weeks that should make building cloud-based applications easier.

One is a so-called Select API so developers can use a familiar SQL-like query language when constructing queries and building applications. The Select API should cut the training time required for developers new to SimpleDB.

The other is a Batch Put facility to streamline uploading multiple items or attributes with a single batch API request rather than multiple single-item calls.

A third-party user interface called Simple Database Explorer, available on a 30-day free trial basis, is supposed to upload existing MySQL data directly into SimpleDB as well as create domains, items and attributes.

In case you're curious, SimpleDB data is stored in multiple data centers in the U.S. Amazon anticipates adding other geographies in time. It uses the widgetry itself.

See http://aws.amazon.com.