A Reuters article, Twitter to launch business tools by year-end, seems to me the best way for Twitter to monetize the service.

It will be “simple stuff” such as lightweight analytics, (co-founder Biz) Stone said. He reiterated the company’s commitment to keeping the service free for everyone…

Even lightweight analytics could be a very powerful tool for those of us using the service professionally and certainly worth some fee since it would likely allow you to track ROI when combined with a complete Internet marketing strategy.