Arabica beans have less caffeine than robusta. In addition, arabica has a much better, smoother flavor, with little or no bitterness.
The stuff at work and eateries, as well as most of the branded coffee in the groceries, is either cheap robusta or a mix of robusta and arabica.
You can get 100% arabica coffees in the store for a bit more. I buy a brand of coffee beans called “8 O’Clock Coffee,” and am fond of both their Columbian (a medium-dark roast) and their French Roast (a very dark roast). I like to use the Columbian ground coarsely with a French press and the French Roast ground very finely with my expresso machine for cappucino, although sometimes I do switch them for variety.
Note: there is good robusta. High-quality robusta blends are favored by some for expresso-based concoctions because they do have more caffeine, better crema, and more bite, so that they can cut through the milk. However, these quality robusta blends for expresso are not cheap, and you won’t find them in your local grocery.
Also, the roast makes a difference in caffeine content. The darker the roast, the less caffeine remains, and also the sweeter and less aromatic the coffee becomes.
A medium roast will be less sweet and will have more aroma and a smoother, chocolately or even almost spicy flavor.
Now I told you all of that to tell you this: buy 100% arabica coffee that is French roast, and you’ll have delicious coffee that, while not decaffeinated, has quite a bit less caffeine than your typical fare. Even better, buy whole beans, grind them yourself right before you use them, and brew in a French press with nearly boiling (but not boiling) water for three minutes.
Yummy! 
–James