It's difficult to find a converter for grams and teaspoons because they're two different types of measurements. If you don't need to be too specific, you can get a general idea of how grams compare to teaspoons (and other units as well) by using this Metric Conversion Calculator from the About.com Southern Food Expert.
Mass Vs. Volume
A gram is a unit that measures mass, which is how much something weighs (like pounds and ounces in Imperial measurements).
A teaspoon measures volume, which is the space that something takes up (cups, teaspoons, and tablespoons all measure volume). Trying to convert grams to teaspoons is like converting ounces to teaspoons: every ingredient is going to be different, so one converter can't convert everything.
Look at the difference in mass verses volume on a larger scale. Imagine you have two one-cup measuring cups, a big tub of peanut butter and a bag of powdered sugar sitting on your kitchen counter. Fill one cup with peanut butter and the other one with powdered sugar. If you pick them up, you'll see that the cup of peanut butter weighs much more than the cup of powdered sugar even though they both take up the same amount of space.
Back at the teaspoon level, ingredients with a similar appearance can have different weights. Making the wrong conversion could easily mess up the whole recipe. For example, a teaspoon of sugar looks a lot like a teaspoon of salt. But the sugar weighs about four grams, while a teaspoon of salt weighs six.
A converter for grams to teaspoons would have to have a large database of ingredients. It couldn't rely on a simple mathematical conversion formula like liters to gallons, or pounds to kilograms.
An Example: Grams and Teaspoons of Sugar
People often want to convert sugar from grams to teaspoons because grams are listed on the Nutrient Fact labels of processed foods. It's easier for those of us who are used to Imperial units to picture a teaspoonful of sugar rather than a gram of sugar.
One teaspoon of granulated white sugar is close to four grams. If you buy a bottle of cola with 44 grams of sugar, you would divide 44 by 4, which is equal to 11 teaspoons of sugar. That's a lot of sugar.