Flowing Data Tutorials by Nathan Yau.
Nathan has created tutorials from time to time on visualization of data.
You will quickly learn that visualization techniques + data != useful presentation.
Useful presentation of data requires insight into the data, the purpose of the presentation, the audience who will see it and related issues, as well as the technical aspects of the visualization proper.
These tutorials will give you a sense of the range of possibilities that exist for visualization of data.
Enjoy!