If you like EasyTweets for managing your Twitter account(s) and scheduling the posting of tweets, then you'll love Sendible. It's similar in concept to EasyTweets, but supports more accounts than just Twitter and has a larger management feature set as well. In addition to Twitter, you can also schedule messages and updates for email, SMS, Facebook (messages and status), MySpace, Orkut, Friendster, hi5 and Blogger. There will be more added to this list, making it even easier to maintain your Web presence in its entirety and keep it consistent across multiple services.
In terms of management, you'll need a centralizing hub for tracking all these scheduled messages you're sending out. Sendible offers reminders and the ability to import and subscribe to information such as your Facebook friends' birthdays. Another very helpful tool is the calendar, which allows you to see upcoming and previously scheduled messages sent out across your various services. It would be great if this calendar could be synced with your iCal or Outlook, especially for power users, but Sendible's reminder system addresses the main aspect of this particular issue, with the option of both email and SMS reminders.
The last of the notable features for Sendible is the ability to include targeted ads in your messages, which can be sent to people's mobile phones and other messaging systems. Considering Sendible's overall feature set, it's clear that Sendible can easily be used as a marketing platform that sends messages on a slightly more personal level.