A few of my favorites:
Beethoven's Moonlight Sonata
The CIA World Factbook
- most excellent for getting political information, such as the legal system of one country, their major imports, exports, etc.
A Metric Conversion site
that allows one to convert any (American) unit of measurement to the Metric system, and vice versa. Pretty self-explanatory.
Those are a few of the ones I keep in my browser (the one above tabs) for quick access. Use them wisely, Ahtonauts!