Rating Projectile Description
Potatoes Pretty standard. Get the job done.
Ice Slugs Difficult to make, but give an awesome shatter impact effect.
Fruits Seal a little better in the barrel than potatoes
Bouncy Balls These little things fit near perfectly into most barrels.  The only problem is that they don't shoot straight.  They often take a corkscrew flight path.
Model Rocket Engines Very difficult to fire properly, but give a spectacular effect
Glow Sticks Awesome at night time. Either strap them to a ball or dart or even drop in after a potato.  These are particularly hard to lose at night time.
Snow  Certainly gives you an edge in a snowball fight.
Tennis Balls Work very well.  Require a special non-standard barrel size.
PVC Dart An excellent recoverable projectile.  Takes a straight flight and flies just like any model rocket.
Water (Pneumatic Only) Gives a really cool effect.  just pour water down the barrel and POOF!  An enormous cloud of water particle is fired.  Only disadvantage: gun kicks a massive recoil!!!
Advanced PVC Dart The best projectile that I have found so far.  Flies perfectly straight and is easily recovered.  Best for testing the max range of any gun.
Note:  I personally have not tested all of these projectiles individually.  This is a compilation from many sources that say that these things work.  If you still don't see anything that appealing to shoot on this list, look at the SPUDROCK site!