1. twAitter – Users can select from a personal or business account and both are free. Sign up is simple; just enter the Twitter username and password. In just seconds, a simple and clean interface is displayed showing pertinent Twitter information including followers, following, updates and more.

twAitter features:

• Schedule Tweets in advance and also select end dates
• A handy calendar is included
• Automatic RSS feed
• Schedule Tweets to repeat on a daily, weekly, monthly or yearly basis
• There is no URL shortener with this service.

2. TwitBabble – TwitBabble is a free download. Users can send messages to Twitter straight from the computer desktop.

TwitBabble features:

• Tweet directly from the computer desktop
• Create scheduled messages
• View and send direct messages
• Search
• View followers and replies
• ReTweet
• Share photographs with others
• Shortened URL service
• A small icon is placed in the computer system tray for easy access.

3. Twuffer – Twuffer offers a no-frills service with a simple interface. No registration is required to use this service; simply enter the Twitter username and password to begin.

Twuffer features:

• Pre-Tweet hourly, daily or monthly
• Includes a handy appointment and milestone reminder
• Users can only send Tweets on the hour and no re-occurrence option is available.

4. FutureTweets – Users can register in two ways; select a username, password and email or simply use OpenID. Email validation is required prior to using the service. The FutureTweets interface is user-friendly, clean and simple.

FutureTweets features:

• Tweet scheduler
• Repeat Tweets on a daily, weekly, monthly or yearly basis
• Send an upside-down Tweet just for fun
• There is no URL shortener with FutureTweets.

5. HootSuite – To begin using this free service, register, add Twitter account information and wait for confirmation email.

HootSuite features:

• Tweet scheduler
• Can use multiple Twitter accounts at the same time
• View pending Tweets and statistics
• URL shortener
• Hootlet bookmark applet
• Ping.fm integration
• RSS/Atom feeds
• Email notification option
• View DMs, Stats, Faves, Search, Sent and Pending Tweets from the control panel
• Generate revenue by embedding Google AdSense code into Tweets
• No re-occurrence feature is included in the service.

6. TweetLater - There are two versions of TweetLater – a free and paid professional. To start using this service, register with a username, password, email address, real name and time zone. A step-by-step tutorial is also available.

TweetLater features:

• Multiple Twitter accounts option
• Save and reuse draft Tweets
• Track keywords
• Send automated DMs to new followers
• Publish within 60 seconds or a specific time
• Includes an automatic URL shortener and tracker
• Automatic follow and unfollow options
• Direct message purger
• Free newsletter
• The re-occurrence option is only available with the professional version.