@Ty Carlson - Technically adding anything to your regular diet is a supplement
To be fair to the OP - The best supplement for mass really depends...
Caffeine/grean tea work great for many guys (both small and large). That new supplement from Biotest will do an even greater job for nutrient partitioning (pricey though, but quite likely worth it due to the short duration required for moderate results) <-- everything comes down to $/mg/effective dose.
Whey protein, fiber, maltodextrin/dextrose and/or flavored weight gainer (quality varies greatly and can make this option as healthy and correspondingly costly as well, convenience is always a factor) make a great choice, as do EFAs (fish oil, flax, and borage oil, etc).
Creatine works very well for many, noticeably well for most (>75%), and low-no undesirable side effects for most, various minerals are needed in greater quantities for all athletes, especially zinc and magnesium
BCAAs (timed peri-meal helps with glucose control (iso-leucine -- you can google it).
Many others, but these are top choices.