TwitterFeed is one of the most popular online tools for Twitter. This free service lets you add RSS feeds to your Twitter updates.
However, TwitterFeed is a victim of its own popularity. Complaints about the system crashing are commonplace, for example.
Read more about RSS to Twitter (php script), a Much Better Alternative to Twitterfeed
EDIT: If you don’t want a php script you can use Engator – great hosted service that makes it easy to auto-update your social networks.