What's the antonym for Schadenfreude?

Seeing just the title of your question ("What's the antonym for Schadenfreude?"), my answer would have been "Mitgefühl" (to keep it in German) or "compassion" (English), since I'd say that Schadenfreude is the absence of compassion.

Having now seen your description as wanting a word to express "joy or pleasure derived from the happiness of others", I'd say "Mitfreude" would be it in German, and "sympathetic joy" would be the closest I can think of in English (couldn't find a single word, though a bit of googling did turn up "Mudita" as per cornbread ninja's response, so +1 from me).

Edit:

Mitgefühl means "sadness derived from the sadness of others".

Schadenfreude means "joy derived from the misfortune of others".

Mudita/Mitfreude means "joy derived from the joy of others".

Mudita is the Buddhist concept of joy.

It is especially sympathetic or vicarious joy, the pleasure that comes from delighting in other people's well-being rather than begrudging it.

I'm not familiar with mudita, but this 1871 definition says it's the meditation of joy, but ... not the joy arising from earthly possessions. Personally I'm not sure it's really an English "word", nor does it seem to be (in its original sense, at least) particularly associated with vicariously experiencing pleasure directly experienced by another individual. In its Buddhist context, the priest aspiring to it should already have transcended concepts of individuality anyway.

I don't know a "proper" single-word term, but empathic joy and empathic pleasure are reasonably common collocations (the first much discussed by psychologists, as the empathic-joy hypothesis).

As the Corndog Ninja noted, mudita is the concept of finding joy in the happiness of others.

If you want a rough German antonym of Schadenfreude (or simply schadenfreude in English texts -- "enjoyment obtained from the mishaps of others," as Merriam-Webster defines it), then Seligkeitfreude would work.