Tweriod is, quite simply, a tool which shows you how many of your followers are online at each hour of the day. It organises this information into Weekends & Weekdays, captures all @mention activity on your account, then breaks this all down into hourly figures.
From this information, Tweriod then offers you suggestions on the best time of day to tweet for maximum user exposure.
As discussed in a number of other blog posts, analysing this kind of data will help you to build a geographical profile of your followers,and then schedule your tweets accordingly.
The only limitation on the free version of Tweriod is in the frequency at which you can request account analysis (at the time of this review it was one request per month).
For a free tool, Tweriod really gives you some great insight. I would definitely recommend giving this baby a test drive!