One day I felt like I haven't much to do in my SP world on Minecraft, so I started this study. I killed 300 of each of these mobs (100 for each test), to see their drop rate. These are the results:
Test 1 - 93 Gunpowder
Test 2 - 107 Gunpowder
Test 3 - 103 Gunpowder
1.01 Gunpowder per Creeper.
Test 1 - 100 Arrows and 93 Bones
Test 2 - 114 Arrows and 109 Bones
Test 3 - 98 Arrows and 100 Bones
1.04 Arrows, and 1.0067 Bones per Skeleton.
Test 1 - 103 Feathers
Test 2 - 92 Feathers
Test 3 - 111 Feathers
1.02 Feathers per Zombie.
If you have a mob in the game you'd like to know the drop rate for, just post it in this thread and I'll try it out.
This is an incredibly old thread, from before even the full release of the game (November 2011), and they show the drop rate for creepers right at the top (do people ever even read previous replies, or any anything other than the title? At the very least you should always read the first post, and it is quite sad how more than a single line of basic text is TLDR for so many people these days).
However, the basic mob drop rate of 0-2 items (uniformly distributed) has never changed, so the average is still 1 (Looting adds 1 to the maximum so with Looting III they drop 0-5 gunpowder, averaging 2.5). You can also always look it up on the Wiki:
When killed, creepers drop:
5 experience points, if killed by the player or a tamed wolf.
0–2 gunpowder. The maximum amount is increased by one per level of Looting, for a maximum of 5 with Looting III.
1 music disc at random (excluding Pigstep, otherside, 5 and Relic), if killed by a skeleton or stray.
1 creeper head, if killed by a charged creeper.
Yes, they don't say what the average is but unless specified one can assume it is a uniform distribution; average = (min + max) / 2; compare to the drop rate of diamond ore mined with Fortune, where they show the amounts dropped for each level since it is not just a simple average of the min-max.