The company yesterday launched Apps for Your Domain, so small businesses can use Google’s hosted email, calendar, instant messaging and web site building tools with a single admin and provisioning console.

It’s an extension of a private beta test of Gmail launched in February, in which users could outsource their email to Google, but keep it under their own domain name.

The company’s plans do not stop with yesterday’s launches. At least one for-pay version is in the pipeline, and more applications will likely be added in future.

We do plan on offering a premium version later this year that focuses more on larger enterprises, Matthew Glotzbach, head of enterprise products at Google said.

There’ll be some additional admin capabilities, APIs to interact with existing user directories, higher support levels, potentially an SLA, he told us. Perhaps most importantly, we’ll be offering it without the ads.

For both versions, Glotzbach said that Blogger and Google Groups are candidates to be added to Apps for Your Domains, as well as other technologies yet to be released.

The barriers to entry for these new applications is pretty low. For the email and calendar service, you need merely sign up and point your domain’s MX record to Google. For the web site builder, some toying with the CNAME records is required.

The services are being positioned as most suitable for small businesses and universities. While free to use, Google’s advertising is displayed on the interfaces.

Glotzbach acknowledged that Writely, the hosted word processor, and Google Spreadsheets, both of which are currently offered separately, could end up on this platform.

Google offering hosted office productivity applications is being seen by many as a direct challenge to Microsoft Office’s dominance of the space.

I think in a lot of ways we see these as complementary, especially when you’re talking about Writely and Google Spreadsheets, Glotzbach said.

But he added that there will be situations where users could choose hosted apps above their traditional desktop equivalents.

They add the key missing element of collaboration that these offline desktop versions lack, but in some cases they could replace these desktop apps entirely, he said. In cases where it makes sense and in organizations where model would be applicable, you can decrease the cost and admin burden by using these hosted services.

While hosted productivity application services are pretty unusual, at least in terms of the big vendors, Google’s current Apps for Your Domain services are not without a serious amount of competition.

Microsoft, for example, offers Office Live, which gives small businesses a free domain, email accounts and web hosting. Yahoo’s years-old small business services offer similar functionality, with different levels of paid-for service.

And, of course, practically every ISP and domain name registrar out there offers such tools for prices pretty much any budget can afford.