1. Find Different Types of Files. Simply add filetype:pdf (or switch pdf to ppt or doc) after your search terms.
For example: filetype:pdf filetype:ppt filetype.doc
2. Find Materials From Educational Institutes. For times when you need a reliable source, add site:edu after your search terms.
For example: filetype:pdf:site.edu
3. Use it as a Dictionary. Simply type define:keyword into the search engine.
4. Get the Time. You can find the current time in any location by searching for time location.
5. Find out the Weather. Search for location weather to get the forecast in any location.
6. Find Live Commentary. To get the score of a game or sporting event, type in a keyword, such as cricket, while the game is going on.
7. Do Calculations. You can enter everything from simple arithmetic to trigonometry using the search box.
For example, you can enter 26 * 9000, or sin(90) / cos(90).
8. Convert Currency. Google has an inbuilt currency converter. Just enter 1 USD in EUR (or whatever currency you need).
9. Check Your Stocks. Simply enter stocks: XXXX.
10. Find Images of Faces. To find image results of people only, add img type=face to the image URL.
For example: http://images.google.co.in/images?q=happy&imgtype=face
This is a very interesting site if you enjoy techy stuff. Some of it is over my head but there are some excellent tips to be had. I have subscribed.