Examples of isotopes are O-16, O-17 and O-18. These isotopes can be used in forensics, but are even more accurate in their ability to tell whether a certain rock originated on Earth, Mars or even an asteroid.
Oxygen isotopes can also tell how the oceans have been heating up or cooling down over eons.
Carbon has 15 isotopes, and carbon-14 is famous for being able to tell the age of organisms. Because C-14 isn't taken in by dead matter, and because it has a half-life of about 5,400 years, archaeologists can use it to date fossils and bones. Also, a person born before the Test Ban Treaty has more C-14 in his body than a person born after the treaty went into effect. This is because of the amounts of the isotope that were released into the atmosphere from nuclear bombs.
Other useful isotopes are cesium-137, which is used in cancer treatment. Krypton-85 is used in the indicator lights on stereo systems, washing machines and other appliances. It's also used to measure the levels of pollution and dust in the atmosphere and to measure the thickness of plastic, rubber, paper and other materials.
Tritium is an isotope of hydrogen and is used to make things such as clockfaces and wristwatches glow in the dark.