The American Economic Association maintains a long and growing list of online resources that will be useful to everyone in the economics profession. Whether you're looking for lesson plan ideas, advice on grad school applications, upcoming conferences in your field, or data for a research project, you can find it here. Great new resources are appearing every day.
The Penn World Tables are the definitive source for real national accounts data. The national accounts for each country, initially in their own currencies, are adjusted using detailed price data to obtain real national accounts in a common currency (U.S. dollars) across countries. These data are invaluable for making comparisons of Gross Domestic Product across countries, and for all manner of research on development and growth.