Twitter is testing a feature intended to make it more convenient and secure for end users to include shortened Web addresses, or URLs, in messages they post via the popular microblogging service and via its third-party applications.

AVG Technologies is offering a free software tool to protect Twitter users from malicious sites that lurk behind shortened URLs.

Little more than a week after Nambu Network said it was shutting down URL-shortener tr.im, and just days after the service was reactivated, its founder announced he would take the code open-source and give up the attractive URL.

The Canadian developer who operates tr.im brought the URL-shortening service back online yesterday, just three days after shutting it down.

Hoping to deal with a growing problem, Twitter has quietly introduced a feature to prevent users from posting links to malicious Web sites. But security experts say that it can be easily circumvented.