Geography
Gothos - A Geospatial Librarian's world.
A great place to start if you have requirements for geospatial data presentation. This site has tutorials and free data sets you can download and use. Works great with Excel, PowerView and PowerMaps.
Directly go to the data page >>
Global
World Bank Open Data.
Free and open access to data about development in countries around the globe. This is the place to find GDP, Health, Environment or Environment data on a global level.
Directly go to the data page >>
Calendar
Multi-purpose date table generator.
This little Excel tool was created by Colin from PowerPivot FAQ. It allows you to generate a date (Dimension) table which is used in almost every BI analysis model. A must when you require to do time serie calculation like Year-to-Date, Moving Anual Totals etc.