/g/ makes useful software on occasion for example. after 4chan pioneered webm as looping gif replacement (long before imgur doing the gifv thing) they made webmcam and webm4retards for example. The latter by its mere existence also highlighted github's overzealous application of community policies.
A bunch of indie games started their life there too.
And then there are derivative and mashup works like fanart, translations.
I only visit a few boards, so i'm probably missing a lot.
But we could apply the reverse Sturgeon's law: in any sufficiently large community, 1% of content will not be crap. 4chan is not special, it's just large.
But it's size together with the temporary aspect of it's content greatly accelerates the iteration process, culling bad content and refining the good one.
A bunch of indie games started their life there too.
And then there are derivative and mashup works like fanart, translations.
I only visit a few boards, so i'm probably missing a lot.