Salesforce.com has previewed a new rich Web platform for building on-demand business software.

Showing its usual talent for catchphrases, the San Francisco-based firm is calling Force.com a free “platform as a service” that will enable even nonprogrammers to easily create attractive online applications that connect with others in the growing Salesforce.com ecosystem.

“Once you provide the openness, we think you will see an explosion of apps analogous to Facebook,” said Ariel Kelman, senior director of platform product marketing at Salesforce.com.

In addition to 50 objects created by Salesforce.com that Kelman said can be dragged and dropped like desktop widgets, developers can create and edit code in HTML, JavaScript or Adobe’s Flex language.

“We want to keep the simple things simple and make the complex things possible,” Kelman said.

Applications can have their own look-and-feel, independent of those offered by Salesforce.com. And while they can be hosted for free on Salesforce.com’s servers, developers can also create their own URLs.

Force.com will be demonstrated at next week’s DreamForce conference in San Francisco, with a preview version made available to attendees. For non attendees, the developer preview will be available sometime in the fourth quarter.

Kelman said Force.com differs from Apex, a Java-like programming language the company rolled out at its Dreamforce conference one year ago because the latter is aimed at developers building the guts or business logic of an application.

Salesforce.com also runs an on-demand software marketplace called AppExchange, where it hosts on-demand software created with partners.

All are attempts by the company to create a thriving ecosystem of software that will help its flagship CRM service better compete with established multi-application vendors such as SAP, Oracle and Microsoft.

At its conference call in August after its second quarter earnings report, Salesforce.com said that after a year and a half, AppExchange has 48,000 registered developers working independently or for 348 independent software vendors that have created 642 applications.

By contrast, since launching its Facebook Platform in May, Facebook now has more than 3,500 applications. The system’s success has been credited with helping the social networking service overtake and pass early leader MySpace.com.

For software vendors or companies with existing Web applications, Kelman said that much of the code can “slide” straight into the Force.com environment without much rewriting. However, business logic and other under-the-cover code will need to be rewritten to hook in properly with Salesforce.com’s CRM service or with other third-party software hosted by Salesforce.com, Kelman said.

“Porting over the user-interface elements you can do very quickly, but there will be some effort involved in migrating the database and logic. But after that, the ability to change your application becomes so much more flexible,” he said.

Besides letting developers host their applications for free with Salesforce.com, the company is working on building an e-commerce engine called AppStore Checkout to make it easier for developers to sell or rent their software, Kelman said. AppStore Checkout will be available in “the coming months,” he said.

Developers have started demonstrating how Apex applications can be used offline through the Google Gears tool. Kelman declined to comment when that capability would be available for Force.com applications.