Web-based office suites are coming into their own at last. For quite a while, Web-based suites -- which offered word processing, spreadsheets, presentations, and other tools associated with desktop office suites -- were extolled not because they did these things well, but because they could do them at all. But the three major competitors, Google Docs, ThinkFree, and Zoho, have all made major improvements in recent months. They're becoming both broader, with more applications, and deeper, with more features and functionality in existing apps.

The question is: Are these three applications really ready to take on a desktop-based heavy hitter like Microsoft Office?

True challengers to Office?

Microsoft Office (primarily its Word, Excel and PowerPoint applications) has long been famous for including every possible feature, no matter how obscure -- and for imposing a hefty load of code on your hard drive to provide all those features, not to mention the heavyweight user interface it takes to support them.

Early versions of Web-based productivity suites tried hard to imitate Office, but they were at a double disadvantage: They didn't offer anything like the feature set of the Microsoft applications, and they were severely handicapped by what it was possible to do in a browser (in controlling the on-screen display of the text sizes and attributes, for example).

A couple of things have happened over time, however. One is that the programmability of browsers has been radically improved, beginning with AJAX techniques. Support for standards has improved as well (though there's still a lot of ground to be made up here), so that advanced tricks with cascading style sheets, for example, work more dependably across the available browsers and provide much better on-screen rendering of the documents.

Another change has been the spread of the open-source software movement. Desktop competitors to Microsoft Office, such as OpenOffice.org, have begun to get some traction. These suites may not come with all the features of the Office apps, but they don't come with its price tag, either. They also offer good functionality, good support for Office document formats (as well as truly open formats of their own), and you pay whatever you want to pay -- or nothing at all. As a result, users have become more open to considering alternatives to Microsoft's ubiquitous suite.

The contenders

While Google Docs, ThinkFree and Zoho vary in the breadth of the applications they offer, their features and their usability, they are all capable of doing real, useful work. They do what you expect of productivity apps -- create documents, spreadsheets and presentations -- in sophisticated fashion.

Then they take advantage of the fact that they are Web-based to add another level of productivity. In various ways, they incorporate "presence" features that let you enable collaboration with others from within the apps themselves -- you can e-mail files, share access to files (either read-only or read/write) with individual contacts or groups, or publish files (to a blog, a Web page, or a select group of contacts).

All three of these Web-based suites are free, and an account includes storage for your documents (ThinkFree and Zoho offer 1GB; Google doesn't specify a size limit, but it lets you store up to 5,000 documents and 5,000 images online).

And because you work in a Web browser, they're cross-platform applications by default: You can create a presentation on a Linux box at home, edit it on a Mac at the office, and display it on a client's PC. Support for mobile devices is still in its early stages, but versions of some of the apps are available for smart phones as well.