Rollers are an underestimated training tool for any time of year, not just the off-season. In addition to a gain in fitness, you'll improve your bike handling due to the constant demand to pedal, balance, and hold a line all at the same time.
A trainer is the easiest way to ride your bicycle indoors, or just stationary. Trainers can be used to avoid nasty weather or enable you to keep riding after an injury that would prevent you from riding in the "real world" (ie. broken collarbone).
Power Meters are all the rage among the racer crowd. However, access to real-time power measurement can be the difference between accomplishing your goal or falling short. That, my friends, is something we can all benefit from.
Zone training is the most simple approach to reaching your fitness goals. Knowing your unique heart rate zones is critical to establishing benchmarks, and getting the most out of your workouts.
Global Positioning Systems can be found on just about any moving object that requires navigation, including bicycles. Whatever type of workout you enjoy, be it swim, bike, or run, a GPS will track your every move.
The typical cycling computer will give you basic information such as speed, and distance. More complex computers can incorporate cadence (rpm), heart rate, power, and more.