Salesforce.com announced Database.com, its hosted relational database service, in December. Since that time it's clear that Salesforce.com is far from alone in the market for offering stand-alone, cloud-hosted databases. There are at least four other competitors, with more on the horizon. And one of those is a company all too familiar to Salesforce.com: Microsoft.

Although customers have been able to install Oracle or MySQL in on commodity cloud instances for years, these services all provide databases specifically designed for the cloud.

In reverse alphabetical order:

Xeround

Xeround, based in Bellvue, WA, offers its own elastic database service based on MySQL. Today it announced that its service will now be available from Amazon Web Services data centers in both Europe and North America. Customers can choose whichever location is closest.

In addition to offering multiple geographic locations, Xeround announced that over the course of the next year it will begin to offer other cloud providers such as GoGrid and Rackspace. Xeround's database is host agnostic, so customers will be able to migrate freely between providers.

Microsoft SQL Azure Database

Microsoft offers SQL Azure Database as a standalone service.

In a report on SQL Azure, analyst firm Forrester wrote: "Most customers stated that SQL Azure delivers a reliable cloud database platform to support various small to moderately sized applications as well as other data management requirements such as backup, disaster recovery, testing, and collaboration."

SimpleDB

Amazon Web Services has its own NoSQL cloud database service called SimpleDB. We've covered it occasionally, both in our "Is the Relational Database Doomed?" article and in our "3 New NoSQL Tutorials to Check Out This Weekend."

SimpleDB is really, well, simple. But it could be useful for very basic use cases. It's also free for minimal use.

Google AppEngine Data Store

As we reported, Google's own cloud database just made a new type of datastore available and revised its pricing. This was also one of the services we covered in "Is the Relational Database Doomed?"

Database.com

Database.com is based on the same technology that powers Salesforce.com's flagship CRM service. That means it must be a robust and reliable database. Database.com isn't available yet, but it's already been field tested for the past decade. It's worthy of your consideration for that reason alone.

ClearDB

You might remember ClearDB from our "Cloud Startups to Watch in 2011" series. Like Database.com, it offers a hosted relational database.

CouchOne

CouchOne, the sponsor company of the NoSQL solution CouchDB, offers a free CouchDB hosting service. The service is still in beta.

We've covered CouchDB several times, notably in this article: "Why Large Hadron Collider Scientists are Using CouchDB." We don't know much about CouchOne's hosting service yet, however.