8 x 8 System: Similar to 10 x 10 but you perform 8 sets of 8 reps instead of 10 sets of 10 reps, still with minimal rest. This isn't a specialization program though; it's used for the whole body.
6 x 6 System: The same could be said here: perform 6 sets of 6 reps with minimal rest.
10-8-6-15 System: For each exercise you perform 4 sets (still with minimal rest):
First set = 10 reps with 50% of what you'll use for your set of 6 reps
Second set = 8 reps with 75% of what you'll use for your set of 6 reps
Third set = 6 reps with as much weight as you can handle for 6 good reps
Fourth set = 15 reps with 35% of what you used for your set of 6 reps
Again, only one exercise is used per muscle group.
15 x 4 System: Perform 15 sets of 4 reps of one exercise per muscle group with minimal rest.
Important note: In the 10 x 10, 8 x 8, 6 x 6 and 15 x 4 systems, you should use the same weight for all the sets. Hypertrophy will be stimulated via cumulative fatigue and not the load per se. If you can complete all the sets, you increase the load at the next workout.