Does anyone know what kind of fruit this is? (olive-sized plum, pics inside)?

I am almost certain that these are damsons. The damson is a very distinct variety of plum (Prunus domestica), with a quite different flavour. I'm surprised noone else got this. I knew it just from your description, which was very good btw!

http://en.wikipedia.org/wiki/Damson

Listen to shitzu girl. Prune is a truly retarded answer as a prune is indeed a product which can be made from plums and not a kind of plant. She could also be correct in her identification. When a plum pit grows naturally into a tree it often reverts to a wild type, with smaller fruits, like its European ancestor.

They are "wild plums" I made jelly/jam out of some last year.

Whoever has said that they are prunes are WRONG... prunes are DRIED plums!... so don't listen to them!

In Austria we would call it "Zwetschke" (don't even try to pronounce it) very popular here, my mother has a tree in her garden. Very tasty.

Curious Orange gave the English name - so no need to repeat it.

Curious Orange is absolutely right!

@ Martin S: I think the spelling is "Zwetschge". We also use that word in Bavarian dialect.

That would be a prune. Smaller than the average plum, but just as tasty.

it could be "duhat", a filipino fruit that taste sweet/tangy... talso called black plu or java plum... though you said that the flesh is orange/red...duhat has white flesh... just a guess...

beach plum?

it's plums you just have such a bumper crop that they are dinky.

i thought they were blueberries at first but they dont look like em when you look closer. They are probably some foreign fruit.

Prunes?

EDIT: Yes, they are prunes.