Under cabinet Lighting is a great source of light to add to your kitchen.  It provides task lighting for your counter tops to make them ideal for food preparation and cooking, it illuminates the back splash on your wall to create accent lighting, it can also be used as a nightlight for the kitchen.   Here are some different types you can choose from.
1) Fluorescent under cabinet Lights:  Are the most energy efficient in providing a given amount of light.   They give off very little heat.  Most cannot be dimmed but they can be a wonderful, inexpensive option. 
2) Xenon lights are the most popular.  They are not as energy efficient and do give off some heat but they do have full dimming capacity, and they give off a nice warm light, similar to incandescent.   Even though they are not cool to the touch, they produce still significantly less than a halogen bulb. 
3) The most energy efficient option would be LED lights.  Althiugh most cannot be dimmed, some do however have two levels of brightness.  Leds are cool to the touch and will last for 50,000 hours before needing replacement.  The initial price is a bit more expensive but if you consider your future savings in energy, led start to make sense. 

We generally recommend the linear style under cabinet lights rather than the puck lights.  The linear provide uniform, well distributed light for the entire counter-top and eliminate shadows.   The puck lights tend to create pools of light in a more focused area.  Either type is fine though.