Your ship's engines may be your most valuable assets. More than just propelling you from place to place, engine mods can be used to decrease your travel time, which is ideal for traders. Other Engine Mods can be used to increase your mobility, making your ship difficult to target.
The most infamous engine mod is the Myriad Drive. It's rumored to allow instantaneous travel between locations, but it comes at a price. The last pilot to use it fried most of the mods on his ship using it.