I would have to say that Trainer Road has probably been the most beneficial training tool for me over the off season. The site offers interval workouts that are based on percentage of your threshold values, so it is really pushing you to improve. Their "virtual power" is a neat idea, but since I don't have one of the approved trainers, I used my heart rate instead. There are even plans that cover 6-8 week training blocks, so it was like having a coach writing stuff out for you. And the best part was that some of the workouts talk you through pedal stroke drills, cadence workouts, and strategies for riding -- all the things that I had almost no clue about.
What you'll need:
1. laptop computer
2. ANT USB stick (I had one from my Garmin 405)
3. A Garmin that supports bike cadence and distance
4. Cadence/speed sensor
6. A towel, a fan, and lots of water
What an outstanding program! Highly recommended for people who want to improve their biking in the most efficient way possible.