Here are links to some interesting datasets and APIs you may want to use for your projects. You are by no means limited to just these, of course.
Google's March 2010 list of popular data searches (great for inspiration).
If you're feeling adventurous, you can also form a project around multiple datasets. For example, how do international travel levels from a country correlate with the strength of that country's currency?
If you find an interesting data source that only provides .xls files, you may use Excel to export the data to a .csv file only. Please perform all data analysis and visualization programmatically. This may seem like more work, but dealing with code is far better for reproducibility and provenance than a graphical spreadsheet, and code is much more efficient when the data becomes large.
Seen other interesting datasets? Please let me know!