Thanks to the effort of the research community at /r/theSilphRoad we now finally have a initial table of PokeStop drop rates and potential drops. Last updated on 04.08.2016.
Do note that this data is not conclusive for levels higher than 24.
PokeStop drop rates
|Item||Total drops||Implied Probability|
Note: Eggs are not going to be considered toward this analysis because it seems they do not count toward the normal item drop number. The minimum number of items a PokeStop can drop is 3 and we never found an egg as third item, but only as fourth or more.