Effective 'equipment minimal' leg strength training for cycling?'
There's been a number of questions about how to improve once you've plateaued with your current training regime. None of them specifically deal with weight training but they are all about improving strength and endurance on the bike.

The commonly accepted answers (and usually championed by wdypdx22) is Interval Training.


  Interval Training.
  
  It's a type of workout where you alternate bursts of energy with
  periods of recovery. There are many variations of interval training.
  If you want cycling specific intervals, you could try "The Time
  Crunched Cyclist" by Chris Charmichael. The book is primarily about
  training for Centuries or racing, but if you follow the methods,
  you'll get into great condition. Interval training is an excellent
  cardo workout and a great alternative to jogging.
  
  One popular method of interval training is the "Tabata" method. This
  uses 20 seconds of ultra-intense exercise (at an intensity of about
  170% of VO2max or 90 - 95% of your max heart rate) followed by 10
  seconds of rest, repeated continuously for 4 minutes (8 cycles).
  Typically, this is done 3 or 4 days a week and not on consecutive
  days.