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Node & Scala haven't even defeated their predecessors yet. Another obscure functional language?

Go is neither obscure nor that functional either.

What are some well-know open source projects/products that are using it?

Here you can find a list of some commercial organizations / products using Go: http://code.google.com/p/go-wiki/wiki/GoUsers

An example for a somewhat known open-source project is probably Canonical's JuJu.

My sense of 'well-known' might be a bit skewed, but let's say docker and heka.