Amy-Mae Elliott, Mashable, compares five tweet schedule tools (Mashable) from April 2010. Some of these may no longer exist or could have been renamed.
1. Twuffer (best for minimalists it says)
2. LaterBro (best for Facebook fans)
3. Twaitter (best for international reach)
4. Future Tweets (best for anyone up to no good)
5. Tweetsqueue (best for chatterboxes)