Many sites offer access to data sets you can use to practice with or mash with your existing data.

• Koordinates is your one stop shop for geodata: satellite images, transportation, utilities and more.

• At the Data Market, you can access "100 million time series from the most important data providers, such as the UN, World Bank and Eurostat." 

• Use Google Refine to "clean" your data sets.

• Semantifi provides access to datasets found within the deep Web, enabling users to "ask simple questions or keywords to search multiple databases, get relevant answers, see automatic data visualizations in real-time even from multi-terabyte databases."

• At Ed Data Express, you have access to "some of the important data that the U.S. Department of Education collects from the 50 states, the District of Columbia, and Puerto Rico."

• Make use of Google's Public Data Explorer to gain access to a wide variety of sets.

• The Data Masher has a variety of public data sets that you can use for creating visualizations.