There are many web tools and applications that have the function of scheduling tweets.
Twuffer(twuffer.com/): Only one function - scheduling tweets, no extra, simple, no need to signup.
FutureTweets (http://futuretweets.com/): Need to sign up, but can choose not showing tweets are sent from the site.
Twaitter (www.twaitter.com/): No need to signup, very intuitive interface, and offer URL shortening
SocialOomph (www.socialoomph.com/) need to sign up, but have many extra functions including automatically follow or unfollow those who do the same to you, create extended profiles to display more personal information, save drafts to reuse, and etc.
Hootsuite (hootsuite.com/): It is one of the most popular twitter apps. It has many features including scheduling tweets.
CoTweet (www.cotweet.com/): Responsive, very smooth interface. Its most unique feature is the ability to assign a tweet to someone to follow up on, which is useful for businesses. However, it does require sign up, but you can manage up to 5 accounts with it.
In addition to above, you can also try ObjectiveMarketer(objectivemarketer.com/) ,Dlvr.it(dlvr.it/),Tweetings(www.tweetings.net/).