Support for extensions to customize Chrome, the top-requested feature for Google's browser, has begun arriving for adventurous users.

Previously, extensions worked only for those who enabled the feature with a command line switch. Now the feature is enabled by default in the developer preview version of Chrome on Windows, Aaron Boodman, the Google engineer who oversees the extensions work, said in a blog post Wednesday.

"We're ready for a few more people to start using extensions--the kind of adventurous people who populate the dev channel," said Boodman, who earlier in his career developed the Greasemonkey tool that permitted extensive customization of Firefox.

Work is already under way for some popular extensions, including Yahoo's Delicious for social bookmarking and Xmarks for bookmark synchronization.

Extensions are enabled by default only for Chrome's Windows users right now. "We've also enlisted some help to get extensions up to speed on Mac and Linux," Boodman said.

The extension-programming interfaces have been changing, and more changes are coming to the user interface. Those who want to start developing extensions should look at the new Chrome extension documentation.

Chrome extensions are written with the same programming technologies as are used for Web pages themselves--HTML, CSS, and the like. Mozilla has a related extension effort called Jetpack under way for Firefox.