Tagging has taken off in the Web 2.0 world as a way to organize and find items of interest. And for the most part they've existed only in the browser view of the Web.

In the upcoming Mozilla Thunderbird 2.0 release, tags will move to the e-mail world, too. The new feature could well serve to further Mozilla's e-mail efforts as it strives to achieve the same success with e-mail as it has with Web browsers.

"The most requested feature we had come into the release was message tags," Scott MacGregor, Mozilla's lead developer on the Thunderbird project, told internetnews.com. "We've seen tags take off in the Web world with people tagging images on Flickr and posts in blogs, and users want that same parallel in e-mail."

Mozilla's Thunderbird 2.0 enables users to create any number of tags and to apply any number of tags to an e-mail message. Developers have also added a history navigation feature, including the back and forward buttons, which MacGregor said he borrowed from the browser side of Mozilla.

Though Thunderbird is a desktop e-mail client, Mozilla understands that there are plenty of users who will want to use it with a webmail-based account. So it has enabled easy account integration for both Google's Gmail and Apple's .mac webmail services.

When a user creates a new e-mail account in Thunderbird with the account wizard, it lists Gmail and .mac as options. Users only have to enter their e-mail information, and Thunderbird will actually figure out the other mail settings, such as incoming and outgoing server names.

Though Google is a strong supporter of Mozilla, that fact doesn't have any bearing on why Gmail support is being worked into Thunderbird 2.0. For one, MacGregor uses Gmail.

"As far as the integration goes it was not developed by anyone at Google," MacGregor said. "That was done in the community, and the data to do that is all publicly available."

Though Thunderbird 2.0 extends Mozilla's e-mail efforts, it is still lacking at least one item users may want: a calendar. MacGregor said Mozilla has a separate calendar effort and an extension called "Lightning," which can add calendar functionality to Thunderbird; however, the effort is still in the early stages.

"We're thinking about ways to bundle Thunderbird plus Lightning as an option at download time for people that want integrated calendar," MacGregor said.

While it is easy to think of Mozilla Thunderbird as a replacement for Microsoft's Outlook Express, which also lacks a calendar, that's not always the case.

"We're a really powerful e-mail client. I don't know that Outlook Express is the main competition for us," MacGregor said. "There are a variety of offerings, including webmail, that users like to use and Thunderbird is in the mix."

Another e-mail client that's in the mix is Eudora, which at one time was the most popular e-mail client in use.

Eudora, now owned by Qualcomm, is now in the process of moving to a Mozilla Thunderbird base of its application. MacGregor said his team has been working with the Qualcomm people. The first goal for Qualcomm is to take Thunderbird and give it a unique Qualcomm Eudora look and feel.

"Once they do that they will start working with the community on core feature enhancements that could benefit all Thunderbird users," MacGregor said.

The Thunderbird 2.0 release is many months behind the release of its Mozilla cousin, the Firefox 2.0 Web browser. MacGregor's not fazed.

"We've always been out a little later than Firefox and the release schedules aren't coupled together."

A release candidate of Thunderbird 2.0 is out this week with the final version expected before the end of April. The final version will be available in a few weeks.